I'm (mostly) trained to fly Marauders but haven't finished due to the costs vs. a standard T2 fit BS. Having established that I don't fly them, can someone please explain to me how this proposed revision helps in PVE? How about in PVP where they'd likely get immediately primaried and alpha'd off the field? To me the whole concept seems to be a convoluted, overly specialized answer to a question no one asked.
The difference is in the Bastion module and added MJD bonus.
The Bastion module is similar to a siege module, except instead of increasing damage output it increases damage application capability (tracking, range) and provides mostly the same bonuses otherwise. It makes them tankier, but forces them to sit still.
The MJD bonus makes short-range weapons much more viable on Marauders - at least, aside from the Vargur, which already has a respectable 50-85km range with 800mm ACs (close-range ammo vs Barrage). It means that the pilot can literally blink around the field fucking things in the face with high-DPS fits that otherwise would have trouble applying that damage. MJDs have a standard cool down of 180 seconds - on Marauders this will be reduced to 54 seconds. If the player has trained the bastion module skill to 5, that means you can hit the MJD, land, do a single bastion cycle, then MJD elsewhere and repeat.
On the other hand, this will also work well with long-range weapons - blinking out to range and then sniping with sentry drones is already a great use of MJDs in PVE. Now imagine it with 1400mm arties, or 425mm Railguns, or Tachyons.
(tl;dr, you are vastly increasing the damage projection, tank and mobility of these ships.)
When it comes to PVP, you do raise valid points - while there may be some interesting 'niche' solo or small-gang fits I don't really expect to see Marauders becoming a common sight in fleets. They will, however, work well for POS bashing in highsec, I think.
It's like I joked in OOC last night:
Transrauders, more than meets the eye ♪
Transrauders, Dreadnaughts in disguise ♫