At lower levels, you can easily get by just by not getting hit (fairly simple to get an AC that matters), and your concentration checks when you do get hit.
But later on, when you're getting whalloped for 80 points, your Concentration will simply fail far too often. You NEVER want a Heal to fail a concentration check, because that almost always means someone dies as a result. Frequently that someone is you.
Plus, if you haven't experienced the uh...joy...of an un-Quickened Mass Heal, well, try it and see how quickly you wish you had Quicken.
Lastly, when jump-casting, it's nice to be able to get the entire spell off while in mid-air. Easy with Quicken, probably not (for most spells) at regular speed.