Thermalling gliders are usually filtered out by the software because they have virtually no groundspeed. A glider that is quite close (say 30 miles at reasonable altitudes) to an antenna will usually be displayed as a primary target (just a little cross) thatīs moving veeery slowly.
These primary targets are easy to miss on a busy day and working with a large scale.
I think that a transponder should not be mandatory for gliders. They are very useful though in some cases though, especially around airfields without protected airspace for IFR-traffic like Niederrhein near the Dutch/German border or a couple of military airfields. Flying in the middle of nowhere you donīt necessarily need a transponder.
What happens on nice weekends is that the 7000-codes will be put into a code-filter and only be displayed in a dimmed colour so that you will still be able to see your IFR-traffic.