Nearly every single animation in the game would have to be revamped so that the animations wouldn't look ridiculous if they added height sliders. A tall sim kissing a short one would really just be making out with the Sim's forehead as opposed to their mouth.
If people want their Sims game to have a lot of depth, meaning a lot of things to do and try, that's already a lot of animations there. Now adding to that animations for Sims of differing heights, that's a WHOLE LOT more animations. Even if you did just the Tall option alone, that's twice the animations of the whole game and added DLC.
I wouldn't expect such a thing in this game or the next. But there could be some experimenting with it in TS6.
Now personally? I would really like a height slider, anything to give my character the depth needed to make seeing him grow entertaining. But I would definitely rule out its possibility of appearing in this game, at the very least.
EDIT: Maybe not a slider since how would the game detect your Sim being tall as opposed to being short? I'd say set heights, short, average, and tall with a hidden trait identifying your Sim as such so the game knows what animations to play.
But I don't know anything about game design, I'm just speculating here.