I find they were very useful in my last match, because of the "spokes" map type. On planets with lots of entrances, I went the route of spider turrets and a fortress or two. This was very effective, but in larger attacks, a little slow to kill every thing. The Forcefields were especially useful in the early game when I was being raided a lot, but lacked the power to do much about it. They helped me get my economy off the ground. I say definitely keep them with an upkeep cost of 1 or 2. I think a one off cost would be too easy a choice to make.