Because I don't have a guild with my LSM, playing it is for the campaign is intensely frustrating. I'll sometimes play for hours before getting in a single win, and frankly, I just don't have the time to waste that much time not contributing to that 10 wins. With Eldar or Chaos, I can easily get there in a couple hours of play in one setting. For LSM it's generally 3 or more losses per win, which means that the longer the campaign goes, the more likely I am to ditch a game full of people who don't listen or cooperate. During normal play I don't care, but the way campaigns are set up, and because I simply don't have the time to grind away for hours for 2500 RTC, dropping matches just makes more sense. I really want them to change the system though, at least for personal campaign contribution goals. A system based on your match performance points would be better, IMO, as I wouldn't feel like it was a waste of my time to stay in a losing match when I only have a couple hours every other day or so.
I generally leave when I join in matches that are already beyond saving to save time and nerves. And I guess many others do that as well. But the campaigns influence this in no way. If I was in the game from the beginning, it takes a lot to make me leave (mostly "oh shit, I am supposed to be at [insert important appointment] in 20 minutes" or no rhinos on Blackbolt attack).
