This was originally going to be a reply to quirel's post stating that forerunner machines shouldn't even need arms. I was going to reply saying, "sure they do, because they look cool", but then I realized I was making a larger point and it was better suited as a new topic.

People on this forum (myself included) often fall into the pitfall of thinking everything in a fictional universe has to appear functional and be canonically consistent. But that's dumb, and it's a philosophy bound for disappointment. The point of a fictitious universe is to entertain the audience, and it's a lot easier to do that when you're loose with your in-universe rules.

So yeah the mantis doesn't make a lot of sense, but it isn't supposed to. It's a fun walking tank. Halo has always followed the "rule of cool" anyway - just look at the freaking scorpion.

Maybe it's been previously established that forerunner machines can make all the holograms they need. Whatever, arms still look cool.

Spartan Vale is ludicrously curvy and that doesn't meet my definition of what a realistic supersoldier should be. Too bad, lots of fans enjoy fanservice. Who am I to begrudge them that?

When it's distressing to you that the chief's aremor keeps changing and there aren't any explanations: too bad - he's gotta look cool because it sells games.

I'm definitely butthurt that the sizes and designs of frigates isn't consistent between halo game installments, but sometimes you have to take a step back and realize that you're never going to get what you want (canonical consistency, that is). Ever. The fact that the games (and other media) are so inconsistent in that regard should be enough evidence that to most consumers, it doesn't actually matter. at all. so we're just not their target demo, and we need to be ok with that.

Or maybe you enjoy being disappointed, you masochist.