I usually just leave if its game that won't have any penalties on me such as a TF2 match or crappy CS match. I mean if the match is no longer fun due to constantly being dead and making no contribution then what is the point of playing it? However, if its a game like COD or Halo, I stay till the end if I can. Its better to lose with dignity in matchs like that. Also I sometimes come back from my losing streak. But there was this one time on Halo: Reach on a Spire Invasion Slayer match where we were winning and I was doing really well and I was lucky to pick up a Sparten Laser. So I'm wasting a bunch of Elites and have one shot left. Then a warthog spawns in one of those circle things you capture for like 20 seconds so that equipment appears and a elite gets into the gunner position. So I line up my sights, charge and fire at the warthog, empty of team mates, and with a bunch of elites around it when all of a sudden an ally jumps in front of my scope as I hit the warthog. I immediately panic but since I cannot do anything he gets killed and its listed as a betrayal. So he decides to kick me from the game via the betrayal system. So yea sometimes I can't complete a game if I get kicked due to an accidental betrayal. btw for anyone that is interested, I went back and checked the aftermatch report later and it turns out the match went downhill for us after I was kicked.
But back to your original question, if I did ragequit I would probably say "Well this game was spiffin chaps until shit got frivilous. Cheerio, pip pip and hope its smashing for whom ever wins."