To me, this is the point of force fields. I've thought about this off and on, and yes they are really brittle, but I kind of like the fact that you have to really put more effort into either a) not letting enemy ships get more near them, or b) putting them under serious protections. I think that adds something to the early game on the lower difficulties in particular, too. Otherwise there is just no chance of a loss at the start unless you crank the difficulty way up.
Amusingly I've never had a problem with this until I started playing against AI10's, since the lower AI's (at least AI7 anyway) have a harvester vendetta, and they also have much smaller assault waves so you don't have so much of an issue with killing them off before they do too much damage (and you tend to have more ships, etc at any point in time too since the game is much easier, so that makes the assaults even more easy, relatively speaking).
With Tech I forcefields costing 23k (compared to the 1.5k cost of a space dock) and taking 4 minutes to build (compared to the 1:40 minute cost of a space dock), and they can't be accelerated (unlike a space dock...), and they get destroyed pretty much every assault wave (just like the space dock), they're pretty useless for defending anything except the orbital command station (and maybe your stack of generators, since the AI doesn't seem to deliberately target them unless there's nothing else to hit.
Late game I know they're a bit more valuable for things like particularly heavily attacked choke points, or defending an advanced factory, but they don't seem to be much use for defending "common" things given their expense, and the amount of firepower the AI and wield on a high AI difficulty.
Incidentally, the reason my command center went down in that previously described incident was because the forcefield I had placed had been taken out in a previous wave, and the time taken to build up the 23k resources required had resulted in the AI's next wave attacking when the new forcefield was only half built, thus the rather unexpected shock.