Everyone hates a turtle. By building defences you just taunt players into wanting to crash you with IPMs. They'll see your 100k+ deuterium stock piled and realise you don't play very often. They'll be happy to set up another planet nearby to you, scan you every now and then, stockpile IPMs and then just blast your defences. Once they've missile-attacked you, you become a farm and may as well become inactive.
The simple thing is, by investing in defences, you don't invest in mines or fleet. And it's mines and fleets which increase your income.
Well, yeah I guess if you want to turtle due to RL that's fine. I have a hectic RL at the moment, and it's pretty easy just to fleet save, farm a few inactives and maybe scan a couple people - time permitting.
A general rule of thumb, only build as many defences as to protect your over-night production quota. Unless of course, you are going to be a turtle. 90% of your unspent resources should be sent onboard your fleet, and the 10% that is produced from then until you next sign on will be vulnerable to attacks. So, obviously in the later game, you'll want to invest more heavily in defence.
If you are defender, don't get lasers. Get like 5000 missile launchers, a large shield generator, and from plasma/gauss in the quantity of 50, once you become in range from bombers and battleships.
Cruisers tear through missile launchers. whereas Destroyers will annihilate Light Lasers. By the time you can afford to invest heavily in Heavy Lasers as your fodder, Bombers are already common. Long story short, due to the nature of the oGame universe, there will always be someone bigger than you out there. All defence does is deter certain attackers.
Edit 1: Also, attackers have the advantage of choosing their fleet composition. Once you've invested in your planetary defences, they are susceptible to anyone who scans you. So, for this reason alone, attackers always have the upper hand. If your defence is rocket launcher heavy, they will send a cruiser-heavy fleet to attack it. If your defence has few anti ballistic missiles, they will bide their time and stockpile interplanetary missiles. If your defence is full of gauss turrets+ they will send battleships and fodder. For everything else, there's Death Stars and Bombers. The idea of defence in oGame is a false economy
. The best thing you can do is to make yourself an unattractive target - through spending your resources and fleetsaving. That way, anyone who wants to attack you - will end up spending more deuterium than they will gain.
Edit 2: As you can probably tell... I hate turtles!
None.