I don't have my instrument ticket quite yet, so don't have the benefit of your IMC flying experience. But I'm not sure I agree that you can not get into structural problems in a severe turbulence or CB's even when you're flying within certified parameters. It seems to me that the majority of structural failures happen in CB or turbulence. Now, they could be a result of overspeeding, overbanking etc, but not all of them. I think there's a grey zone here, and because the victims of structural failure never live to tell about it, and we don't carry data recorders, it's not something that can be processed in statistics.
Just this morning I got AOPA safety foundations new accident case study about a Cherokee Six that breaks up with 5 people on board in a storm in Texas last summer. It's a marginal decision to continue at night with so much bad weather, but honestly, with such a well equipped airplane you'd be forgiven if you'd made the same choice as he did. Have a look:
http://flash.aopa.org/asf/acs_timela....mc_id=F912SP3
Did he overspeed? Did he overbank? Probably, but we don't know that for sure. There is a remote possibility that the CB was severe enough as to cause structural failure even if he was flying within design parameters. So, the lesson one must take from this and countless other flights is that flying at night, in, or close to CB's, is a bad idea. It's better to take the long way around.