They probably won't stop a determined attacker. However, they will stop an opportunist from attacking an undefended planet without a portal.

I build them to prevent easy raids just like Arby said. That being said though I know players who demo them once the port comes in.

usually ur amount of lasers is 2-3% ur amount of buildings, that's not a big loss. u could raze them to replace them with expensive buildings(since that will be OB) once u are fully portalled.
The thing is that if an attacker brings ur ground to nothing, a banker with 0 bombers can finish the raid...
Having 20 or 30 lasers everywhere wont stop any attacker with 30k or 50k bombers, the best is to have different amount of lasers on ur plts.
If u put 30 lasers everywhere an attacker will need like 32k bombers to take 100 plts. If you put 5 lasers on half your plts(the smallest) and 55 on half (your biggest) an attacker will need 60k bombers to take 100 plts. And you will have spent the same amount of resources than building 30 everywhere.

explain evilrunt...

IMO - if you're going to build 20/30 lasers per planet, you may as well not have any.  Let me explain.

As an attacker you almost 100% expect ur enemy to have 20/30 lasers per planet,  thats standard.  So already you have failed to be unpredictable.

Lasers are useful early in the round, sure, but later they become a nuisance.  If your ground is high, you have set up effective anti raid fleets, your not fully portalled everywhere,  you have a secondary fleet stationed and your active,  then your need for lasers is reduced. 

However, If your not as active,  i suggest you go extreme and have 40 lasers per planet.  it will lower your planet loss each round by about 60% garunteed.  Be smart about where you explore and keep track of your enemies.

Lasers are OK, but any big fam can jump 10k bombers on each player, bankers included without breaking a sweat. 

So go high or don't bother.

Orbit is correct.

A few rounds ago, when I was extremely planet fat (over 1k planets) I was avg of 40 lasers per planet. A good idea is to build 1/3 your planets a low laser count, 1/3 a middle count, and 1/3 a high count. Mix them all up. Confusing an attacker is key. Make they waste more bombers per fleet.

A couple people have said these points already, but let me sum things up.

In short:
1. Always vary lasers counts. NEVER have the same amount of lasers on all your planets.
2. Always mass build lasers across various planets. NEVER build lasers on planets individually.
3. Don't go overboard with lasercounts. A determined attacker will still get through.
4. Use alternative ways to prevent yourself from getting raided. Do not rely solely on lasers.
5. NEVER fully portal (100%).
(6. Always build lasers. Otherwise people without bombers will be able to raid you)

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In depth:

1. Always vary lasers counts. NEVER have the same amount of lasers on all your planets.
Try not to become predictable. If you have the same number of lasers across all of your planets, you become an easy raid target for an attacker: he can just send the same (small) number of bombers to all of your planets, and grab them easily.
As Kollop said: "If u put 30 lasers everywhere an attacker will need like 32k bombers to take 100 plts. If you put 5 lasers on half your plts(the smallest) and 55 on half (your biggest) an attacker will need 60k bombers to take 100 plts. And you will have spent the same amount of resources than building 30 everywhere."

2. Always mass build lasers across various planets. NEVER build lasers on planets individually.
Infils are an attackers best friend. An attacker will love you if you build lasers on planets individually, as his infil will plainly show him how many bombers he has to send per planet to fail none. Always stack several layers of lasers on top of each other, using different groups of planets to build different numbers of lasers. Make those infils extremely confusing to attacks, so that they will have to waste ops on random PI's. Combined with point 1, they may still not get the complete picture with the random PI's and still fail tons of attacks.

3. Don't go overboard with lasercounts. A determined attacker will still get through.
As Arby3 said: "What they will do is slow down attackers. Bombers are cheaper than lasers to build in mass quantities. Laser trapping that planet to 5000 lasers will cost you WAY more than the guy who's building 50k bombers. It's also less useful. "
Not only are those lasers costing you more than it is costing the attacker to jump bombers. The attacker will be able to just use the bombers to raid tons of planets, they are not at all a drawback for him. He will now just be able to hit more planets at the same time, making him perfect for doign the cleanup job after someone has ran ground.

4. Use alternative ways to prevent yourself from getting raided. Do not rely solely on lasers.
As Orbit said: "Lasers are useful early in the round, sure, but later they become a nuisance.  If your ground is high, you have set up effective anti raid fleets, your not fully portalled everywhere,  you have a secondary fleet stationed and your active,  then your need for lasers is reduced.  "
Anti raid fleets are where you station a part of your fleet on a(n unportalled) planet, so that when you get raided while you are offline, and your ground is getting ran/has been ran, an attacker can simply take that planet from you. This will cause the remaining fleet that was stationed on that planet (after fleet losses) to be recalled to the main fleet, preventing the opposing attackers from taking your remaining planets with small fleets. As an attacker, it is a delight to find out you can trigger those 50 droid fleets you'd sent and grab massive amounts of planets with little to no fleet. Do not let this happen to you!

5. NEVER fully portal (100%).
This is to prevent your ground from being ran easily. If you are portalled on 100% or even 95% of your planets, it will be very easy for opposing attackers to send big fleets to enough of your portals to run your ground down to nada. Remember, every time an attacker takes a portal, he can kill up to 15% of your main ground fleet. 2 max kills and you're down to below 3/4 of your original ground. 4 max kills and you're almost down to 1/2. Leaving planets unportalled may make the attackers who work less securily or lack op support (in the form of IP's) to waste big fleets on unportalled planets, possibly leaving them stuck with hovering fleets that they cannot trigger because your ground hasn't been ran down enough. Alternatively, it can make attackers waste fleet on spreading out big fleets across multiple planets to ensure that they hit a portal. In both cases, it means you're giving them a harder-than-usual time.

hehehe fair point although I doubt many people are that dedicated to keeping infils up, but there may be some!

My advice for early in the round is to build soldiers and station.  dont go crazy with lasers just keep building them slowly,  keep your infiltrate busy.  Soldiers are better for long term defence.  once ur lasers are killed they are gone... bad investment.  once you lose a planet with soldiers, you still have some left to fight another day.