I 've played a few MMO's where you had to do something to get your skills up.
Usually, it meant i had to log in once or more a day and expend my "action-points" which tends to get boring after a month or so,
the whole having to log-in/play on a regular basis to stay competitive becomes annoying and the grind becomes repetitive,
at the same time if i wanted to play the whole day, well, there was interaction(communication) with other players but these games tend to lack other types of content.
Time-based leveling seems a good system in accomodating the player.
Off course, this model works best in a pay-to-play model, or a pay-to-level-up model,
as to discourage inactive characters just to grind levels.