Removing /assist was one of my "dreams come true", unfortunately it was implemented quite a while after I quit. Even if there are multiboxing "cheatz0rz" you can still have the upper hand in pvp just by playing smart... my 2cents
The old-style tower/tara PVP was like: "mash da assist buttonz, pew pew". Mass PVP was not about being a good pvper overall, but only your ability to press the assist macro and faceroll your attacks. Wars were decided almost entirely on how good the callers were while was generaly not fun for anyone else, especially docs who practically never survived any battle if the callers were any decent.
I'd say FC is not responsible for people cheating with 3rd party programs. You find a true multiboxer (not just some dude who logs in other accounts and juggles the gameplay with Alt+tab - which I think is perfecty fine and one of the better features of the game) report him. Simple. Don't blame FC.