If you’re an actual professional photographer shooting portraits, you’re almost always tethering for maximum control over the final product. We’ve all heard the arguments that for years pro photographers only had one roll of film per shoot. Same applies here, if you fear of a card failure, and you don’t tether, shoot multiple cards. I always tether on shoots because the memory is so much larger, and I don’t have to worry. Additionally, if you’re working with a team, which most pros do, they need to also see the output on a larger screen so they can spot things you as the photographer may not see.
If you’re willing to pay extra for a card slot, then so be it, but the lack of one doesn’t not make it a consumer/prosumer level camera. Seriously, there’s nothing I can’t do with the R that I can do with the mkiv. However, there are multiple things I can do on the R that you can’t do on the 5D.
I’d mostly be interested in a body without an AA filter for those circumstances where I don’t need to deal with repeating patterns or fabrics. The 30mp in the R and 5D IV are already plenty.