Expo has a role, and high expo has a role as well. It's completely dependent on how you fly and what you fly. When used properly, it keeps the plane controllable while still allowing rapid and extreme control throws. I have used expo as high as 80% for some routines on planes that required 60-80 degree control throws for particular maneuvers with much less extreme maneuvers before and after. Normally, I would switch to very high rates and back just for that maneuver, but if the routine flows in and out, that isn't always possible.
As a general rule, I use 15-20% for low rates, because somewhere in that range is necessary to produce true linear movement of the control surface from a rotating servo horn. For high rates I start with the greater of 20% or half of the control surface throw in degrees, then adjust as necessary for the flying I do with that plane.