Spells spent for defense are helping you survive, and survival is job one.
If I'm a Wizard who gets stuck in melee my goal changes from "help to win the combat" to "survive long enough (and by so doing, keep my attackers focused on me) that someone can come and bail me out".
If I would have choosen to play my wizard defensively last Monday, where my group was ambushed by some mean undead, the group would have been TPKed.
The main reason is of course party composition.
No martial, our cleric himself was a ghost (killed the session before ...) and couldn't use turn undead, the bardlock can pew pew Eldritch Blast and the Psi Rogue can Psi dagger dagger ... so I'm the main damage dealer when there are several enemies.
I was also blind, because it was dark and my character is a halfling - so I had to fireball us a way out of there (and shatter and whither and bloomed) and to get some light my wizard burned down a library worth of books ... which had hurt more than the bites of these shadow creatures ...
Like literally, if in this fight the enemies could have interrupted my spellcasting, the whole group would have been dead. We wouldn't even had have a chance of survival.
Our Party composition wouldn't be able to survive most encounters if enemies could easily interrupt the casting of spells.
You would force parties to have martials, even though players don't want to (and in another thread we already made the comparison: In Germany, where I play, Fighters are not popular like in the US, the most popular class is the cleric in my survey in the biggest german d&d facebook group, the fighter was in the middle).
Right now you can play D&D with any single class party and have a fun game a chance of success.
With thile proposed change here, that would be impossible.