Rather than trying to force the logi into a support role by making it worthless in combat, why not make it better at logi'ing (and, while we're at it, making everyone better at their intended role)? Eve Online does a great job of this through ship bonuses. I'll use the
Dominix as an example, since it's my favorite ship.
For Eve players, you know where I'm going with this, but for dustbros, let me explain the reasoning behind using this ship as an example. You can draw on your Dust experience for a few assumptions - It's a Gallentean boat, so it probably armor tanks, is good with blasters, and an put up a decent active armor tank.
Except you'd be wrong with one of those assumptions. While the Dominix is capable of fielding six large guns, it has a ship bonus of 10% per level to drone damage, as well as 7.5% to sentry drone optimal / tracking (the wiki shows 10% - this has since been nerfed). Rather than use turrets in your high slots, you'd want to play to your drone strength and fill your highs with drone support modules (namely Drone Link Augmenters, extending your drone control range), and maybe sacrifice a bit of lowslot armor tank for drone damage amplifiers. I mean, who wouldn't try to fit their ship to take advantage of a potential 50% damage bonus?
Here's another good example -
Nemesis. Another Gallente boat, this time a wee frigate. Looking at its stats, it appears very fragile. Doesn't like this thing would be able to put up much of a fight, right? Normally it wouldn't, unless you fit it in line with its bonuses, making this thing a pain in the ass to fight, even more so if you brought friends with you (yea, you outnumbering this guy makes his weapon even more efficient). The bombs, when dropped right (and even better, in small gangs of these ships), can leave an enemy gate camp fleet reeling in pain while you warp off fapping like a madman with the nerd boner you just popped after dealing more damage with a frigate in one shot than a BS could ever hope to do to the same fleet, while keeping your ship intact.
I'd like to see this system work its way to Dust, where a suit intended for a specific role actually does the role better than a medium with a few mods swapped out,
provided they fit the suit to play to its strengths. Scouts should be able to use profile damps to get their signature down to that of a common space-house-fly, while being able to boost his passive radar to rival that of an active scanner. A sentinel should be able to sit on objective B, laughing off bullets with his armor plates / shield extenders and innate light weapons resistance, until an Assault cuts him in half vertically with his plasma rifle / rail rifle / combat rifle / scrambler rifle. A logi should be a force multiplier, passively enhancing the strength of his squad (new modules ERMAHGHERD (not the Crusader, the Logi)) while being able to provide field repairs and resupply. Just a few examples.
So yea, the most common complaint I see about logi's are that they are too good at everything. Rather than make them useless, make everyone useful
at their intended role. I know this idea might mess with the whole "build your mercenary your way" message, but Eve Online also has some ship fittings that ignore their bonuses and still work wonderfully (passive shield tank Myrmidon, the battle Venture, battle Badger, to name a few). A suit designed for a certain purpose should be able to use the tools that aid that purpose more efficiently than a suit that isn't meant for it, but that doesn't restrict the suit to that one role.
TL;DR - Buff each suit in the area it was intended to be used for by implementing module usage bonuses like Eve. Make a logi want to logi because "HOLY **** this thing reps so much better than my scout!", "GREAT GOOGLEY MOOGLEY, this Assault just wrecked that Heavys multiple anuses (anii?)!", "HOLY SHITTING DICKNIPPLES, my Scout can see everything, and no one is shooting back at me!"