I could kind of use something like this too because I'm starting practice to backing tracks and don't feel like hitting Ctrl + Z every time or not having a count in at all. And yes being able to adjust it to something more than a 4 count would be helpful as it gives some time to get my hands on the fretboard.
And an unlimited or very high number count in would be useful (don't make it limited to like 8 or 16 measures) because here is an example. If I want to practice Little Wing or something with a long intro that has no backing tracks I need the click there to keep me in time until it kicks in but don't want the click when it does. Another element that could be added to this is a way to make two sets of count ins that sound different so again if I'm practicing to Little Wing I can have say 8 bars count in to get myself ready then it switches to a different "ping" sound to signal me that I should be playing and then of course go silent when the backing track comes in.
Basically the metronome could easily have far more functionality than it currently does.
Oh and maybe being able to give it a tempo curve or swing or something. It's basically just a preprogrammed MIDI dealymabob ain't it? Shouldn't be that hard to slap together.
My needs in this area are probably very different than others though as I'm realizing that Sonar is going to end up becoming a practice and teaching tool for me.