Neither am I.
Instead, I am arguing that conventional vehicles are better than mechs in all instances. Or close enough as makes no odds.

: Like you said, its footprint would mean it
: would be more prone to sinking in certain environments. But there's plenty
: of terrain that a tank simply can't go through unless trees are cut,

Alright, I'm being facetious. Dense woodland is an area where a mech (or large power armor) might find its niche. That said, none of the Halo mechs are woodland specialists. They're just walking tanks. Hell, I can only recall one mech across all media that I've seen that was purpose-built for fighting in woodland terrain. It was a picture of a dieselpunk Nazi mountain mortar on Deviantart.

Two arguments against:
-A mech isn't going to bring a tank's armament into the woods. A tank is heavy and prone, which makes it an ideal platform for cannons with heavy recoil. Mounting such a cannon on a mech means that you've just welded it to a top heavy platform that can't effectively transfer the recoil to the ground. Ideal weapons for a mech would be mortars, recoilless guns, rockets, and machine guns, which are good enough for woodland terrain. But then...
-Why not bring an armored car? In Halo, I guess this would translate to an uparmored Warthog or a vehicle of similar size. We've already seen the Warthog with the Mantis's armament*, and I'm DYING for a Warthog with a M490 automatic grenade launcher in back.

In the case of dense jungle that even a Warthog can't get through, I suggest Colonel Kilgore's approach. Fwoosh.

*Can't remember how a M79 MLRS stacks up to the Mantis's M5920.

: bridges over gaps built, et al, that walkers can simply step over.

>.>
So, we're talking about terrain that is crisscrossed with ravines and chasms more than a meter and a half wide*, but less than the three or four meter stride of a Mantis. Also, the ground is packed hard enough that the pilots don't have to worry about losing their footing while stepping across.

*Judging from Loftus's charts, the Scorpion's bogies are about three meters long. I'm just guessing here.

: Mechs *can* cross terrain vehicles can't.

Color me skeptical.

: That doesn't mean that they'd be
: better in all instances, but that's an absurd argument I don't see anyone
: having.

I'm arguing that the terrain that mechs can cross that conventional vehicles can't is nonexistent or rare. Worth developing a whole new class of vehicle to deal with? Probably not.

: While I suppose you would argue that they'd be more prone to tipping (and
: compared to a tread vehicle, they would always be more likely), look at
: how advanced our gyros are now on even consumer tech like Segways. These
: things would be pretty stable scrambling over obstacles even at speed.

You can't credit all of a Segway's stability to it's gyros. A lot of it has to go toward the overall design: a two-wheeled bottom heavy scooter, with very responsive electric motors driving the wheels.

: Obviously they're never going to let you run a Mantis across a map at
: 60kph for gameplay reasons, but that's another matter.

Somehow, I doubt that the Mantis can go 60kph.
I probably have a hard time imagining it because, for the most part, mechs are shown in fiction to be ponderous and hardy. Fast ones in Western media are a rarity. They're a Hell of a lot more common in Eastern media, but those are outside of consideration.

: While I see the argument of MJOLNIR-clad humans in mechs as redundant...

Honestly, I'm glad that the following never made it off the drawing board.

: you
: can say that about Spartans in almost any vehicle. A Spartan will be more
: nimble and faster unencumbered, but A) mechs, tanks, and the chaingun on
: the back of a Warthog offer more firepower, and B)

I don't say it's redundant. I just think it looks incredibly stupid.

: even with the presumed decrease in costs over the years and improved
: technology, it's clear the entire UNSC corps will never be fully or even mostly
: comprised of Spartans in the near term.

Are you kidding?
The UNSC is pulling back from the brink of oblivion, yet they still have enough resources to take 300+ adults and turn them into Spartans, complete with next-generation armor and a holodeck.

What's that? The augments don't work well on post-pubescent humans? We don't know what you're talking about.