IMO the discs any bag will be better with than without, in order of importance:
A beaded (preferably big beaded) putter (Wizard, BB Avair, Challenger, Banger)
A beaded mid (Roc, Wasp, Sentinal) - you'll notice a lot of guys who throw a Buzzz complement it with a Wasp.
A "workable" control driver (Gazelle, Cyclone, Eagle, XL, TL, Cheetah, Polaris LS)
A very overstable driver (Predator, Firebird, Spirit, Banshee)
Understable duties can be performed with a beat version of your control driver.
A faster, stable discs for headwinds is really nice to have, but not 100% necessary.
A Teebird is nice to have, but again not necessary. It's hard to argue with straight and long.
Distance drivers (Valkyrie and faster) are the least useful disc in any bag...unfortunately they make up at least 50% of most people's bags.