Collision Avoidance and ATC

In the US, ATC is only responsible for providing separation services
between IFR aircraft, not IFR from VFR. They will call out VFR
aircraft time permitting. A glider pilot with a transponder might have
the false impression that someone is providing separation services for
them. This could happen as work load and time permit for ATC. So to
sum is up:

1) ATC is not required to inform a VFR or IFR pilot of other VFR traffic.
2) It is the responsibility of both VFR and IFR pilots to avoid VFR traffic.

Even if ATC calls out your position while thermaling, an aircraft
traveling 250 kts might only get a glimpse of you ever mile while you
circle (assuming you are visible when perpendicular to their flight path
twice in a turn). With TCAS, a glider pilot has better chance of
detecting approaching aircraft both VFR and IFR that are squawking codes
and see / avoid them first.