Although you have always been able to play every class on one character, it's always been that you are meant to pick a "main" class for a given raid tier and gear that up. That's how it's been for the whole of the last 10 years. You can still play other classes in dungeons and open world content, and you can even still raid on all classes using crafted gear, you are just slightly weaker that's all.

Other people do think this because it's been debated a bit recently, but it's always been this way and a bit strange that it's only being brought up now.

It could be useful to be able to gear up all of your roles at once, but it could make you indecisive about which role to play as well. Once you've picked one it helps you stick with it.