Well, I would prefer if the AI acted as you've put it rather than how it does now, as it will force the players to actually play smartly. Squishy mages did perfectly fine in D&D based games if you actually used some tactics.
I'm pretty sure squishy mages were ok in the D&D based games because the AI there *wasn't* set to do the sensible thing. They were instead aiming either at what they saw first/nearest target, or --in slightly more advanced systems-- they'd take damage dealt to themselves as part of the calculation.
Later on in a bid to make the AI act 'smarter' that was enhanced with taking into account the healing done to the enemies and that's the point where the AI started to focus the squishies and suddenly players discovered that hey, that's not fun. And so the taunts were added to the picture as a way to allow the classes function like the players expected them to.
XCOM features no aggro mechanics, yet both squishy snipers and beefier assaults have their place in the squad if you use them properly. There are tons of games that somehow manage without it.
And again, XCOM doesn't prioritize your squishy targets from what I remember, which allows them to actually function. It's not 'using them properly', it's basically the AI giving you a break. Much like the AI in strategic games gives you a break by not doing the most sensible thing, which would be ganging up from the get-go on their largest threat, the non-AI, unpredictable human player.