Well RDR basically set the bar. So if Bethesda can't make horses at least on par if not better than those in RDR, then there's no point in having them at all.
Though I do personally believe there are horses in Skryim and they look and play fantastic.
I personally disagree with this sentiment. If every game cut out every feature of it that was done better in another game, well you would get some pretty dull games. Nobody would want to include character relationships in a game because Dragonage did it really good, or have a climbing system because Mirrors Edge had a better one than they could manage. I mean I'd rather have no horses than awful horses, but they don't have to be better than the best (I hear RDRs horses are pretty much perfect) to be considered worthy of implementation. Just good.
EDIT: Those are just random examples BTW, not saying they should necessarily be in Skyrim. But really, the ES games will do certain things better than other games, and some things not as good, but not every feature should have to be the greatest implementation ever.