I know from a UK Airbus pilot that under their procedures they ignore a PROB30 TEMPO in a TAF.
But then an airliner has a lot more options when it comes to icing conditions, and has weather radar, and ready access to any available bit of air they ask for.
As Fuji says, it is absolutely right that one should not avoid flying just because one is "unsure" or something like that. That just leads to giving up flying. Any go/no-go decision should be made on technical reasons alone.
I am no weather expert but as regards forecast CB activity, if I had to fly in the area then I would look at the lowest forecast bases, subtract a bit for good measure, and if the result is still well above the MSA I would go. So if the CB bases are forecast at 3000ft, I would assume they could be at 2500ft, and plan the flight for 2000ft and ask if that is well above the MSA. The MSA does count even on a VFR flight because the vis is likely to be next to zero in a heavy shower. In reality one would fly around a heavy shower but one has to have an OUT option of which a 180 back home is the least favoured.
I would not set off into IMC (i.e. without a VMC on top option, due to airspace etc) if there could be embedded CBs; my stormscope is pretty good but it won't see them until they get pretty serious.
One wouldn't do a scenic flight with a bird on a day like that, though
