You only ult in if you've got a chance to hit a 3-5 target Force Pulse or unload a full combo on a priority target, and have either Flash or Zhonya's to deal with avoiding the reprisal. Otherwise, you skirt the edge of the team fight, picking off anyone who strays away from the rest of the team (a badly damaged support flashing over a wall, the enemy AP who's already blown their CC/Ult and is too far away for teammates to support, etc.) until people start getting low enough for you to just roll your way through them one after another.
You generally don't melee in a team fight unless the enemy team's in complete retreat and you're just having fun with the stragglers. Your range is the range of your ability combo, not the range of your auto-attack.
That's for an actual team fight, obviously. If you're talking about a siege or a "dance", you're mainly just waiting for the moment to capitalize on someone getting caught out of position. Preferentially, someone else on your team does the actual catch - an Elisse Cocoon, a Zyra root, etc. - and you just use your ability to Riftwalk in and drop your combo to blow them up. Then you either follow standard hit-and-run Kassadin team fight procedure (if they respond), or Riftwalk after them and try to slow one or two with a Force Pulse to generate an extra pick (if they retreat).
The in-and-out is where it's at. You've got a ton of damage on just your Q and E because of the way Kassadin's kit is tuned. You don't need to be trying to R onto people or bashing away with constant auto-attacks to completely dominate a team fight. I know, it's counter-intuitive and absolutely not conveyed well by his kit; it's one of the reasons I feel his design is very sloppy.
