All very good and relevant points from Drumshticks and RP3.
For drummers adding here, it's a real 'chicken and egg' situation. Drummers should be the foundation of time all others play to and only when that happens does a track sound natural to me. Of course, that isn't possible on the Loops or we'd just be doing templates all the time! So when adding to someone else's template, the timebase needs to be consistent because, for me at least, I have to 'belong' to the track - i.e. sound like I was there all along - without sounding like I'm just tagging on. With a click to guide me, I have a reliable reference allowing me to concentrate on the feel of the track safe in the knowledge the music is staying in time. Without a consistent or steady timebase, this is almost impossible to do reliably (or without a lot of editing).
As mentioned above, it is the reason I often overlook tracks I consider to be excellent but are just too hard to play well to. If I can't do a track to a quality I'm happy with, I will just move on. I rely on being able to line up my click in Reaper to the contributor's track to help keep me in check. Without it, I'm constantly having to respond to timing variations in the performer's template which ends up sounding like I'm constantly playing catch-up with the track.
Distorted guitar and bass-only tracks are doubly hard without being able to set up a click. Not necessarily for recording quality reasons but because of the volumes I have my headphones set to (especially as I record acoustic drums) causes everything to get very muddy. This is where I need the click to guide me even more.
I realise this is a technical heartache for many players to perform to a metronome or program some guide drums for their recordings but doing so is far more likely to attract real drummers to your templates.