Does there have to be a challenge? There's a lot of species that are much easier to see than rails are to hear, should we discount all of those? The other way around, there's also plenty of birds that are easy to see but very hard to hear...
Good luck on the Yellow rail though, I've heard those are expecially hard to see!
The way I see it, counting heard-only means less disruption of the birds (because you can be happy with a call, instead of being inclined to flush or draw out the bird using pish/playback), more efficient use of your own time, and perhaps most importantly it teaches people to bird by ear, which is an invaluable lesson and step in everyone's birding career. Also, in my opinion sound observations can be just as fun (or even more fun!) than visual observations... But maybe that makes me crazy!
Ah well, I don't feel like discussing it further.
Just putting my opinion out there, because I don't know if it's actually something we've ever questioned or talked about in these threads - would be interested in others' input, but I also don't mind the current ruleset.
