[Steve]

I don't have a least used lens as I carefully chose all my lenses to fill my shooting range from 17mm through to 300mm. There is little overlap in my lenses and I have absolutely no issues at all switching lenses (even for just one shot) so all get used frequently.

I guess if I became more specialised in the future it could change but I probably would not sell any of my lenses even if they where not being used much.