To add to my post, I want to address the title/OP. I don't think Anet is worried if people leave or not, I actually think they were aiming for the casual drop in and out style of gameplay. After all they aren't making a lot of money from the average player so there's only costs associated with having a high number of concurrent users. They'll make money from the PvP events they'll run and people who like having vanity items and spend lots of money in the gem store.
For me, I find I'm currently spending more time on it than I do on WoW. Of course when MoP is released, I'll be on WoW almost entirely at the start to level up, gear up and start raiding. As soon as everything's on farm and I don't need to spend much time in-game, I'll go back to messing around in GW2 + other games. Just like I did with the original Guild Wars.