What makes you assume there aren’t artillery, ASF, and lighter elements with a superheavy unit? They were never deployed by themselves in canon.
Wolf’s zeta battalion, for example, would be a unit where superheavies make sense as well. It’s a brick brought in against the targets nothing else has the meat to crack, and works in canon.
If a target is slow enough to need the firepower from a Superheavy and can be caught by something moving 2/3, that is what artillery is for. One unit fires a
C3 Remote Sensor, and if the target is spotted the data is passed back to the artillery unit. (Can't remember what the artillery-delivered version is called). If the target is not spotted then the Remote Sensor is shut down or destroyed, and the
Expendable Recon unit keeps searching.
(I also figure a couple guys on a cargo truck use a Lift Hoist to pick up and carry the expended 250-kg Remote sensor, so it can be refit for future use. Each one is ~25,000 C-Bills, so it would be worthwhile to recover them. I'm sure there's some sort of canon cargo truck that has a cargo bay and a Lift Hoist that can do this job. Since whatever they are chasing can be caught by a 2/3 Superheavy, this truck doesn't have to be all that fast.)
At least, as long as your force is willing to use artillery. Mech-carried artillery would be able to go nearly anywhere a SH 'Mech can go.
The other option is a bunch of light and fast VTOLs whose only purpose is to probe closer to the target's expected location, using their high TMM to avoid getting hit. Once they spot the target they call in artillery and back off. If the target is mobile, then they will watch to see which direction it goes so the artillery can adjust. If it is not mobile, their job is to try and spot any hidden exits from the base.
The main problem with SH Mechs for me is that as soon as you go from 100 tons to 105, the Internal Structure goes from 10% to 20% of the Mech's mass. Assuming standard Internal Structure this increase means the Internal Structure mass goes from 10 tons to 21 tons. You actually lose capability with a the lighter SH 'Mechs, and the only use I can think of is being able to put enough armor on it to survive a single ortillery shot or a nuclear warhead.
There's one very specific advantage for lighter-end Superheavies: They make good Long Tom platforms. Conventional 'mechs simply don't have enough crits to mount them. They're feasible enough to get to 3/5 without breaking the bank and if that's not fast enough, well, you're not going to find too many Long Tom platforms that are faster anyways.
Good point, you'd have a 3/5 all-terrain Long Tom platform. I completely missed that use, good catch.
I think there might also be some minor mobility advantages associated with superheavies associated with crossing light forest hexes or something else? Most of my rulebooks have disappeared into the void, so I'm going exclusively off of memory here.
IIRC, SH pay 1 less MP for crossing a hex, minimum of 1. So Rough terrain that needs 2 MP to cross for a normal Mech would only need 1 MP for a SH. So if you expect a lot of Rough and Light Woods terrain, then your regular 4/6/0 Mechs will be slowed to 2/3, while your 2/3 SH Mechs won't be affected (ditto for 6/9/0 Mechs and 3/5 SH).
