The need to correct the balance points to a problem when one cabal holds sway. While there is almost always a problem here, Nexus isn't necessarily the right solution, just one possible solution. The underlying problem could be the multiplying affect of taking a cabal item and having numbers on gameplay balance.

An alternative solution might be to give EVERY cabal an ability like vanguard that doesn't require holding their cabal item. So the one or two fighting the gang of six has an ability to stay and fight.

Another solution might be a general nerfing of all skills/spells/supps & powers for members of cabals as proportion of players in that cabal increase. For example, you have 6 members, next cabal only has 2, everything you do drops a level or two, and your offensive melee takes a hit.

I think either of these create some real incentive to log in when you are the underdog.