Is it normal for gliders not to have attitude indicators? This answer shows a picture of and describes a "modern" glider instrument panel, with two variometers, airspeed indicator, altimeter, compass, temperature gauge and radio controls.



Notably (at least to me) missing from this is an attitude indicator (or "artificial horizon"). I had the impression that an AI was one of the more important indicators in an aircraft.

Is it frequently left off on gliders? How about other aircraft? Is this related to always avoiding flight outside of VFR (visual flight rules)? Is an AI not very useful or not useful at all in VFR conditions?
Yes, it is normal. With very rare exceptions, gliders are not allowed to operate in IMC (instrument Meteorological Conditions) and must remain clear of clouds. There are also powered aircraft that do not have attitude indicators, mostly for the same reason (VFR, Visual Flight Rules only).