I see the RTG as more of a "backup energy source", it works all the time, and produces a decent amount of power, but in order to get a lot of power from them you would need a lot of Uranium/MOX fuel cells. For example, to get 16EU/t from 1 RTG you would need 15 plutonium aka 135 Uranium/MOX cells, which would be more then enough energy to last a good while.
As SpwnX said, they are a great portable, use anywhere generator for powering remote bases, quarries, miners, anything that doesn't need too much power and isn't near your main base.
And as for "abusing RTGs as your main power source", I think it would be much easier (not as easy with GregTech) to spam solars or windmills for your main power source then burning through enough nuclear fuel for that amount of Plutonium, not to mention that (like I said above) as long as you are not running a mass fab, that much fuel should supply enough power to run an average base for a while.