In theory, it's an interesting idea, in reality, I think it may actually be one of thsoe things that's more trouble than it's worth (to keep up with, not necessarily to implement). I guess it depends on the size of the company.
In theory (at least to my thinking) issues should go to a department first and then get assigned to individuals, so someone needing to keep track of things like vacations would only be doing so within their department and, presumably, if they have the authority to hand out ticket assignments to individuals, then they would likely know enough of what's going on to know who's not available that week.
I guess I just think that any company big enough to need this feature should also be structured enough NOT to need it.
I could be way off, though, and maybe I'm expecting too much of people. :)
Edit: Hmm... re-reading this now, I'm not really sure I'm articulating myself properly. :/ In a nutshell, I can see it being beneficial in some situations, but I don't know if it would be beneficial enough that other people would use it often enough (with all that entails) to make it a core feature.
