The 50% viability number is not accurate, it's a rough, and if IIRC, pessimistic estimate. JZ may have done his own counts, but I read an article in the journal of the ASBC a few years ago. They only tested a couple of dry yeasts straight from the manufacturer, and going from memory, one was 85% viable. The point is, there are too many factors to say 50% die. And I need to renew my ASBC membership
All of that said, I often rehydrate using go-ferm. Sometimes I direct pitch. Usually it depends on the OG, high OG gets rehydrated. Under 1.050 I don't bother.