Here is why Fascinate is a much more powerful ability in this game than in PnP:
You can do it in battle. In PnP, once battle starts, or anyone even makes a threatening move, poof, fascinate is no more.
Oh, and you need to keep playing.
If fascinate did need any tweak, I would say make it so that the bard toggles it on and off. As long as they play, and the mob is undamaged, viva la song.
Though as many have said, the required proximity and limited number of songs makes this a difficult prospect. If you want a useless ability, try the Suggestion ability. It lasts for so little time, and if the mob gets whacked at all while fascinated, your suggestion is down the drain.