Actually I wondered a little bit why we got 4 character slots in The Division 1. During your progress to the maximum level the character could reach, your choise of the skills got no influence which type of "class" your charakter is.

If you see classic MMORPG you can do the "standard" content with each character solo. If you want to do things like raids or something like that you need people who knows how to handle their character class. With this option it makes sense to got 4 character slots if you want all kind of combinations available for yourself.

Games as Rainbow Six got that kind of feature as well. You combine different skills to a very good squad. And a very good tank with a shield could make a difference.

I don't think Ubisoft will realize that kind of change with The Division 2 ... but I think that will force some people to use all the character slots and will force some people to know how to tank in The Division. The most Division Agents I know just focus on the Damage they can deal and even never met a player who used the tank skills.