If you say that velvia 100 is essentially the same as 50, you haven't put it through it's paces. In many cases they are very close. In others, things get ugly. Mainly red as others mentioned and not just in shadows. I was showing a friend some 4x5 chromes over the weekend and he asked "why are these trees purple (pines with red bark) in this one and not in this one?" Rvp 100 vs. rvp, that's why. I only use it when I need the speed/reduced reciprocity.
Also, I'm not sure what provia has to do with the op's question. Totally different animal.