No, you should NOT quit a game because you are loosing.
BUT! There should NOT be a ban for people quitting either, I am currently banned from multiplayer because of 'quitting'.
I quit one game earlier because ALL of my team quit before me and I was the last one left being repeatedly spawn killed by the other team. After this game I got the warning about getting banned if I quit another game.
Then I loose connection to a game of SWAT and I get banned. Its pathetic, I just bought the Halo Anniversary CE disc with DLC for Halo reach and I feel I have wasted money now. PATHETIC.