I see your point, but I can't see a "realistic" reason for unbreakable teams. It's no different to any other assignment (IMO of course). Of the top of my head, here're some counter-balances for having reassignable team members...
Penalties associated with moving people in and out, as part of their skill levels will be in the interaction with other team members. So a team could have it's base skill level (everyone's totalled abilities) plus an increasing over time skill bonus but then drop that down like when a ship changes task force. If the team is not actively working on a job, then it's bonus skill points either don't improve or deteriate a a given rate.
Add something like the Intrepid Explorer's Peril Possibility for various teams, e.g. cybernetic team activates ancient security systems wounding or killing members, diplomacy team manages a big blunder (skill drop, possible dismissal, execution/imprisonment by locals), etc.
If you put a ranking system on the teams, e.g. team leader Rx others must be Rx-1 or lower, then the teams can be forced to break up after promotions too.
If you allowed multi-skilled teams, set their primary role to one and all other skill areas are half skill. Though I think you're right that would probably be too flexible. Maybe you could have a cost associated with creating a team (support staff, equipment, etc.) and if you wanted it to do a different job it'd have to be "retooled".
Stephen