"The humans aren't as diversified as the aliens, so we should add a bunch of new stuff."
"Uh, but the aliens aren't really diversified either."
"Well then let's add a bunch of new stuff to them as well."
... Wait, what?
Also, the humans have even more versatility than the aliens. With aliens, game play tends to break down into "evolve into highest possible alien evolution (s1: goon, s2: goon, s3: adv. goon/tyrant), attack humans/human base, repeat".
With humans you have a lot more choice in the matter. At S1 you can take a lasgun/mass driver for easy long distance kills on small aliens, or grab a shotgun to stand a chance against goons/own dretches at close range, or maybe even grab a saw and rush the base. At S2 you have all the previous choices, as well as "should I take a jetpack or a battery pack" and "will I need a grenade on this trip out". Should you use a flamer to get some easy creds from stupid dretches, or should you take a pulse rifle to tear apart that goon? At S3 the choices become even more diverse. Do I take a battlesuit, or do I take a battery pack? Should I take a chainsuit or a lucisuit this time to help my teammates the most? Can I even get to where I need to go in my battlesuit, or am I going to need a jetpack?
Humans have a myriad of choices for gameplay. When it comes to aliens, though, it's just "upgrade to highest alien available, attack". Thankfully, the changes coming for aliens in 1.2.0 should help add a bit more tactical aspect to the alien team.