I think I read somewhere that the reason they couldn't do height customization in CAS was because it would make interactions too difficult. Like a tall sim's hands would just "float" above an object when picking it up or hugs/kisses between people of different heights would be too hard to animate. I think this was why they only had a few preset heights, less work to animate everything for every possible height. I'm not sure though. With the new game engine in TS4 it may be different, so I would love to see height be a thing.