Even if the right slide pad remains optional for the entirety of the 3DS'sesses life, any revision will likely include it built in. Nintendo wouldn't build a different sized add-on for each revision when it would be cheaper for them to just make it part of the revision itself and leave it up to developers to support it or not.
Anyway, I'd only buy a Control Pad Pro if it came packed with a game I wanted and that was the only way I could get the game, similar to how Wii Sports Resort was originally sold. I still don't have Resort so that's probably a good indication of how acquiring a CPP is going to play out.