It really is just customization. This logic can be used the same for all armor customization and having different armor can be seen as useless because it doesn't do anything. If that's the case everyone should just be Gray Spartans and only have color distinctions when on teams. Armor effects may be useless, but with this view all customization as a whole is.
Customization is only there for you to make your Spartan or Elite look unique. Nothing more, Nothing less. It's never done anything. However, it is an integral part of Halo now. It extends longevity as people progress or unlock achievements for Armor. It just is what it is. Dress-up with cool armor, make'em look like you want them to look. And I have fun playing with the Super Soldier Dress up honestly, and I'm pretty sure everyone else does too.