Well, it's only undesirable due to the current six month thing for an MPC. But then, hasn't the constant argument in favor of that been that it's a 'soft' time limit, or whatever?
In response, towards hags having a process of being developed over a long period of time, that development is done as a player. Only once achieving the template does the timer begin. As an MPC, you must be a player first and your story must have made sense to lead up to being the desired MPC. All that development is before the six months timer.
Only problem is that even after becoming hags, they'd continue their steady, methodical and intelligent approaches to things. Not the 'leap out of the shadows in front of a crowd of adventurers and go OOGABOOGAOOGABUGGABOO!!!! then run away and hope they don't die immediately' method of play like MPCs right now are, in a nutshell, expected to be. Hell, Scurvy was awesome, but in the end I don't think anyone feared him. It's not exactly fear when someone says 'I think I saw Scurvy!' and rather than people looking worried and fearful, seven to ten adventurers pick up arms to form a lynch mob and go hunt him down. In the woods. In the middle of the night. That's not fear, but that's pretty much what's expected of MPCs as such.
Which is why I make the point of the only obstacle standing in a Hag MPC's way is the fact that they WON'T be the 'leap out in front of you, declaring their nature' sorts. Hell, Hags would probably be the safest MPCs to play due to the simple fact that they, by their nature, are reclusive and keep to themselves, in their covens, only coming out when it's necessary to further their plots and their schemes.