You can use the legacy one, but you won't be able to use sprite animation, if it is a 2d game, and you use some sort of ik for the 2d character (with splitted body parts, and parented inside unity, then animated), you can use legacy animation, there is a thread regarding this.
But I'm all about mecanim right now, for me it is really good, and once you grasp the concept behind it, there is nothing you can not do literally with mecanim. For me there is no reason to not use mecanim, and I don't think I'm gonna go back with legacy again...Is there any difficulties for you with mecanim system?