1) Realism ? Or more accurately Game/SGSeries continuity. Cant move millions of raided UU through a stargate instantly ?
Nor could you move Trills of Naq through a stargate instantly, do you suggest 'no farming with MS' too ? Nor could you fit Mills of UU or Trills of Naq on a ship, nor could a ship attack 1 players homeworld one second, then attack another players homeworld the next second. Do you suugest say 12 hours travel time between MS attacking 2 different players homewords ?
2) Benefit of sending MS out ?
I agree with Lore, rank for G&R and ability to destroy many enemy MSs in hit and runs, denying them the chance to retaliate are the 2 main reasons.
3) Remove raiding all together ?
Raiding was designed to let new but active players to catch up with older less active accounts, relying on UP alone they would could never catch up so it essential for newer players. The down side is raiding created such an abundance of easy UU that the game turned into a way of making $$$$ for some, and became severely unbalanced by $$$$ players. Rock and Hard place scenario. No obvious solution.
@ Bebita, bad Mr Bebita, you want an update to make people keep MSs home, coincidentally days after you get an account with a very big MS

I'd say NO to this idea, most raids are on 0 def players, so no hurry to move the Mills of UU, raids on active players with a def usually yield far less.