If we're talking about ethereal mounts, the remounting process is akin to casting a spell (you'll notice the similarity in the character animations) which is why casting won't allow you to re-summon immediately until the casting action is complete.I'd like to know why you can't remount a pet while you're casting a spell, @Kyronix Intended?
I can't remember when it changed, but I'm sure It was during one of my many Diablo 3 or WoW breaks.