Aside from a quick weapon swap, it's pretty unrealistic to be able to change gear in combat... Even the weapon swap happens way too fast to be realistic.

Of course, that's not an argument against changing abilities.

Having said that, if you could swap them out in combat, you'd get slaughtered when trying to do so. Unless you use an add-on to do it, but then people using the add-on would have a significant unfair advantage over people who don't use it.

Forgoing the IRL argument, I kind of like that you might run into a fight and not know the dynamics, you equip the wrong stuff, you wipe. Then have a rethink and try something new. I mean, I'll admit on several occasions I've been in combat and realized I made a huge mistake with my ability layout. And I have two addons that let you assign hotkeys to armor/weapon/ability layouts, so you can swap among them with just a keypress. But it seems to me, being able to do that mid combat would give players a huge advantage. A single player could actually cast every ultimate for their class, plus Meteor and Flawless Dawnbreaker, within a single fight.

I have a solution to this, but I'm not gonna say it, lest ZOS actually like it and implement it. Because while it solves this, it's pretty much going to enrage most people.