You have a better physics background than me, but it seems like the Philae mission would be particularly concerned with weight because of the velocity it had to

You have a better physics background than me, but it seems like the Philae mission would be particularly concerned with weight because of the velocity it had to achieve. Considering it took 10 years to reach its destination, maybe the shielding required for an RTG would be prohibitively heavy. Maybe the overhead in increased costs due to limited availability of PU238 and additional launch preparations/safety were better spent elsewhere. I figure the ESA made the best decision for the mission, though, and were well aware of the tradeoffs involved, don't you?