X-Men Legends

Inactive characters will level up at reasonably close to the same rate as active team members, until you start using Cyclop's and Storm's leadership skill,which gives a damage boost and an XP boost only to the characters who hit the combo.

You can replay danger room missions to get an XP push to make up any difference in levels for inactive characters.A couple of the missions in particular can get the lagging characters caught up to your main team.

Some equipment can only be worn or unlocked by reaching a certain level, which is the main advantage to sticking with a core team that will reach these levels faster.