I just last night played on a table with built in cup holders, and remarked to myself how it always seemed like my beverage was in the way. Everyone has a different idea of where their beverage should go (in relation to their chips, cards, etc..), in between their arms, on the side? So the real issue for me is that people's ideas will conflict with each other, and someone will have a beverage in their way. Secondly, you're limited to certain positions. If you have a 9 seated table, and move down to 7 players, then it's harder to just spread out, when spots are pre-arranged.
Not saying rail cupholders are horrible, as I've played with them just fine, and it wasn't that big of a deal. I just think if I had a choice, I'd prefer to have slide-unders (that you can place where you wish) or drink carts.
Just my two cents.
Beautiful table mockup tho... Stunning felt!