Well, you only need one place to store everything.
Obviously, your slots your choice but you don’t have to have 4 IF you could switch characters mid battle from one specialism to another character YES that would justify 4 L31’s that could allow you to play different roles as a team. Thinking about it now, its a great idea. Imagine - Vanguard +3 others all at mid 20’s.
You could help them out killing a load of enemies for XP, then as they rank up further - can play a support role spotting enemies, reviving, medical and ammo drops it would be brilliant…but the balance is off on a lot of the skill tree, the effort far outweighs the outcome.
IF the team ever decide to go level out the skill tree, the XP cap, prestige points, re-spec feature will all have to be looked into and would take years to develop.