I've just started flying with a Mode C transponder and have been
alerted a couple of times of approaching traffic when it started
replying to frequent interrogations. I was able to locate these
aircraft visually. Clearly an alert system would be an inexpensive and
valuable asset.

Having the right of way in a glider is about as useful as having right
of way in a sailboat when you're about to be run over by a mega
container ship (which has also happened to me). Large aircraft (and
vessels) just might not see you and, even if they do, can't avoid you
becasue of their mass and speed. I'd just like to steer clear of
conflict, which includes alerting others to my presence and learning
about theirs.

Good grief, there are still folks in the USA (including some well-known
glider FBOs) that fly without radios, let alone transponders.

Mike