I think it adds more replay ability. Like I only lvled up Jack. Now I'm kinda bored of him. So now I get a chance to earn more cards and gear for my other players...Leveling up individual characters instead of classes is way, way too much of a grind. Gears 4 had a much better system for Horde and cosmetics. This kinda sucks in comparison if I'm honest and I was hugely looking forward to it.
If all of them leveled up with my 16 Jack. I would not bother even playing them. I like the grind, I like earning things and feeling the progression.
Working on Kat and other Halo guy now. I think this may be the most I have played of horde on any Gears game.
They just need more maps and more supplies to choose to build with. Give us gun towers. Mortars. Blades traps. Tar traps. More environmental interactions.