If it is a hunting rig, don't bother with a lens, the drawbacks far exceed any benefit.
1) Rain/condensation issues
2) Decreased target brightness
3) Glare/Reflections
4) Reduced clarity of the target => need clarifier......see #2 above
5) The 12 ring on a real deer is as big as a pie plate, not like that itty bitty ring on a 3D target

For a target/3D rig, they can be helpful and generally the above reasons aren't too large of a consideration to gain some extra accuracy.

I shoot a lens on my target rig (long stab, scope, the works.....) but on my hunting and 3D rig (same bow) I use a standard Spot Hogg sight with .029 and .019 pins.