Have to say its quite a good idea but I think you're only taking into account a certain type of player, the general one who plays an average amount per week/month.
The thing is it would be a real kick to those who play irregularly. I'll just make up an example here, take someone who...I don't know...moves around regulary. This type of person may have a month of time to play without interruption but then Bam! They hit the xp cap. Then next month they have to move again and will lose the opportunity to play, thus loseing any chance to level allowing all their friends to race ahead. Whilst without an xp cap they would have been able to keep up with others by playings loads then not at all, intill time gave them a chance again.
That said I don't think this will matter to much as from what I have read so far I get the impression that that game will be skill based instead of level, a bit like Morrowind
.