I'd love a count-in feature (it has been noticeably missing for me too, on more than one occasion). I have almost zero issues with Live, and no real complaints... but if I did, this would stand out as about the only one......
A count-in interface could be very simply added, up there in the control strip at the top of the screen... just two items:
[precount on/off button] [number of bars/beats field]
How hard is that?

So if it's on, you get a pre-count of xx bars or beats, and then it stops (or not, depending on your metronome setting).... where does that count-in start? Well, it could be tied to the punch-in point, if that's turned on, otherwise, wherever you are at the time, minues the count-in value....
I don't see it as being any problem to add (meaning adding undue complexity, or gumming up the workflow approach), and it's certainly "less good" for me not having it in there....
I just think having to 'add a track with a click file in it' is a "workaround" approach, not a real "workflow" method......
I don't know how adding this feature would diminish the value or useability (or intended use) of Live?? If you don't need it, just ignore it! However, with a count-in function added, Live would be even closer to perfect, IMO...
(By the way, there's a big difference between pre-count and pre-roll... I don' t know that we need pre-roll, but certainly pre-count at a minimum!)
just my two-cents and a bit......
peace,
tribalogical