The only advantages your opponent gains are as follows:
1. If he doesn't know when he can get back to the game, he reduces his chance of losing on time.
2. He gets to annoy his opponent i.e. you (if you let him).
As to the abuse issue, if he is complying with the rules then he's not abusing the system, though it may feel like he is, from your perspective. Don't worry, the game will finish at some point, in the meantime, why not start another game as consulation?
I currently have an opponent that switches to vacation mode after every move. Is this person abusing the system? Is there a benefit to this that I am not aware of?