As a player who mostly plays supports, I find myself sometimes being useless when I can't click on my allies in the middle of the combat, because another enemies, creeps or allies overlap his hitbox.

One spell is Warlock's Shadow Words, when I want to save my allies by healing him, instead of damaging enemies. One worse situation is Omniknight's Repel, when I can easily miscast it on enemies instead of allies.

Therefore, I suggest to be able to cast a spell on ally by click his portrait (including when using QuickCast).