AFAIK there are scripts that have so many combinations that Unicode does not have a single codepoints for each character. In Arabic you can have one of 5 vowel signs on each of the 28 letters. But Unicode does not provide 5*28 codepoints for the combinations. That is probably the reason for have these combined characters.

I dunno, they didn't seem to have much trouble jamming 11k or so hangul letters into the standard, as well as the smaller combining forms of them. Not sure about Arabic though.