@FlyingTomahawk We have quite a few very talented programmers working on making ES better all around. All those different mods you mentioned are things that each of their respective programmers are passionate about and want to eventually see as part of the RetroPie core.
In saying that, each one is a pretty major change and needs lots and lots testing before it can become part of the main branch. It's much easier to test these mods if they are in different branches/repos, so you are basically testing a stable RetroPie with just one new mod. If you tried to test a bunch of mods together, then there's no telling how they might interact with each other.
They way I see it, if someone is working on a mod that you really want to see in core, they can help test it. Maybe they're better equipped to test one mod over another. I'm helping test this Carousel mod, both because I really really want to see it as part of core, and because Zigurana has been awesome enough to have a Windows build of each new change.
I'd love to help out with Grid View, but without a Windows build, I'm limited in what I can do (I've tried the Ubuntu virtual machine route, and it was nothing but aggravation).
I'm impartial to Video Previews, but as a theme builder, it's something I need to keep an eye on even it's not something I can help test too much (and I respect the hell out of the work fieldofcows is doing). Also, if I remember right, Video Preview is already part of the latest RetroPie, but I think fieldofcows is looking into a different video player to combat some overheating issues or something.
So basically, lots of mods are a good thing, and the more people like you and I can help out the developers, the faster they can become part of RetroPie core.