where as the panels afford the craft the possibility of functioning indefinitely...
Except for the small fact that they don't expect the probe to operate as it approaches the sun (it's a comet they are on and they tend to "activate" when they approach the sun).
Had you been talking about a probe set to go well away from the sun then absolutely and pu-238 power plant would be a great idea.
And if they actually did expect the probe to survive the approach to the sun, as it is a comet, it would have definitely gone far-far away from the sun...
AFAIK, one of the main reasons they didn't use and RTG is that no usable ones were available from NASA and the ESA didn't yet have the expertise to make them themselves. Not to say that they still wouldn't have gone solar because of other mission parameters (e.g., mass), but the reasons you give don't really add up.
FWIW, once RTGs are assembled, you don't get to "hibernate" them, they start to decay immediately (kind of like a battery). If you don't have the capability to make them yourself, there might be one on the shelf you can buy from someone else, but it's probably not designed with your power budget in mind and it still has it's original weight, but the amount of power it will put out is reduced by sitting on the shelf. It took 10 years to get to the comet, so that's really puts a lot of constraints on using a pre-existing RTG, perhaps too many for the mission, but if they had the technically ability to make a custom one that matched the mission criteria, I'd bet they would have strongly considered it.