*Ahem*

In all seriousness, I can't see the advantage of huge mecha. For instance, to continue paying homage to the Metal Gear series, pretty much every Metal Gear, from Shagohod to REX, was essentially a nuclear missile launch platform with some extra weapons. Well, with the exception of RAY, which was an anti-Metal Gear Metal Gear (lolololo), but hey. Not very practical. You could take the definitive technology- say, REX's giant ass railgun which propels undetectable nukes- plop it down in the middle of a heavily fortified base surrounded by automated defenses and infantry, and it would still probably be less expensive and easier to build than a giant mech; probably more effective, too. There's a point where size becomes a massive disadvantage- your mobility will inevitably suffer, and even if you have ridiculously thick armor, there are most certainly weapons which can punch through it. Initial building costs would be astronomical, as would repair costs. I'd rather be in a fighter jet than a mecha.

Giant mechs and spaceship fighters occupy the same realm of really cool but impractical combat ideas. The thought of them evokes awe and excitement, but their feasibility in real life situations compared with alternatives is low.

Even some exoskeletons are generally going to be a bad idea if combat doesn't allow for the specific niche that it fills. You'd either want extremely streamlined units that are basically more non-enclosing enhancement modules or you'd just want a normal tank instead. The in-between can't stand up to tanks in open terrain nor can it stand up to dynamic, moving troops in dense terrain.

The future of warfare is in arial drones. Lots and lots of arial drones.

Or once everyone got sick of real-life war they all settled down for a macth with Halo/BO/CoD/BC/Another FPS.
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Nah as above we'd pretty much end-up with a CC-like war using tiny drones as our soldiers....minus the 'mining for gold' part.

What about giant moving bases, a-la Metal Gear 2?
Are these in any way useful?

The only ways I can see mechs being useful are as exosuits, or construction jobs. Where dexterity and strength are useful, as well as versatility.

Eh, they're just not very practical. It would cost billions to even engineer and construct one, and they'd no doubt be highly susceptible to damage. The only way I can envision a sort of moving fortress is one that is either air- or sea-borne. Enormous vehicles on land just don't work; they're too unwieldy for practical movement. An aircraft carrier, on the other hand, is practically a floating fortress; it supports hundreds of personnel, has automated defense systems, and carries a supply of fighter jets that can be deployed at a moment's notice. A giant aircraft ala a supersized AC-130 or something would also be more effective than a moving land vehicle, but the problem with those is, while faster and more maneuverable than a ship, they're a lot more fragile. Airborne vehicles are pretty susceptible to damage and mishaps- not that ships are invulnerable, they're just easier to maintain. Aircraft also have to be refueled often; I'm sure it would potentially be possible to create a nuclear powered aircraft, but the cost would be, if you'll forgive my pun, astronomical, and ultimately not worth it.

tldr -> Big ships, and to a lesser extent, huge airplanes, would be effective war machines. Moving land fortresses are probably out of the question.

Huge airplanes would never work, but I could potentially see a huge airship. Nuclear power, well insulated and isolated from crew compartments, could either A: heat a shitload of air to lift the whole structure, or B: pump up water from oceans to get hydrogen. Obviously the hydrogen isn't ideal, but there's no real way to get helium out of the atmosphere, and you will inevitably have leaks.

Hot air lifting seems like it'd take an awful lot of energy, meaning a faster reactor rate, meaning more frequent refueling.

Also, on the subject of giant bases, y'all should look into project habakkuk