Something that's been bugging me.

[Everon]

Eecently we've seen Shikamaru work with other chunnin (granted it was under Sarutobi-Sensei). That's their gentle suggestion that teams often get formed based on needed abilities during a mission.

The eight teams were originally built so that teammates would complement each other's abilities. Along with teamwork experience, it makes sense to use the these configured teams often.