dtb said:

"Or perhaps there would be some science value in risky close passes, like flying close to the rings again, until it eventually gets clobbered, rather than decades worth of Titan observation."

Titan is really interesting. We're currently studying the CN radiation problem for Titan atmospheric entry. Future Titan missions are definitely in the pipeline, although nothing has gotten past proposal. The CN (cyanide) radiation problem stems from CN being a nonsymmetric diatomic molecule. Nonsymmetric diatomic molecules are very efficient at converting thermal energy into radiation. For example, the molecule NO (nitrous oxide) is the big emitter for terrestrial atmospheric entry. The CN bond is really strong so a lot of energy can be pumped into a CN molecule before it dissociates. For example, the peak heat flux for the Huygen probe might(?) be 200 watts/cm^2 where 160 watts/cm^2 of that is due to CN radiation. No one knows how much heat flux Huygen will actually experience because no one knows what the methane concentration is in Titan's atmosphere. The aerothermal guys are all going to be sitting on the edge's of their seats when Huygen hits Titan's atmosphere (it'll be more exciting than the MER entries and that says alot).

Gary