No problems, Robert. I understand you perspective.
I think we live with accepting risks every day of our lives. The quest for knowledge is not without risk.
There are probably more Plutonium-238 bearing RTGs lying around garbage dumps in Russia that probably represent a greater risk to life overall, than Curiosity's. (I mean … we at least already know life exists here.

)
In the earlier times, several spacecraft RTGs have crashed back onto Earth and some people were exposed to the alpha radiation emitted by the damaged units. Shielding and distributed isotope design specs have improved a lot since those days.
If a unit were to somehow partially leak radiation in situ on Mars during the operational life-time of the probe, the radiation detections on board Curiosity would probably detect it (and measure the levels, thereby allowing this to be subtracted from the assessment of the 'habitability').
One of the quests of Curiosity is to measure the amount of incident radiation on the surface. This is likely to represent far more risk to extant life, (if it exists), than the MMRTG also, because incident radiation contains a broader spectral mix of different types of 'nastier-to-life' radiation than MMRTG's alpha type. If life exists on Mars, then it has had to survive in a radiation-bathed environment for a long time.
Overall, I think our pathological, instinctive fears of 'radiation' may play on our emotional side when it comes this issue, whereas the rational side has got it all well-scouted (IMO).
Cheers