Aside from grinding the soles to nothing walking around in the parking lot, do I need to worry about my shells being worn out?
1) excessive wear to the soles from walking in parking lots diminishes the boot's rigidity in the binding. This is like adding lots of base bevel to your skis as the convex boot sole rocks loosely in the binding system before the ski edge engages. (Take care to keep your boot soles true and flat)
2) boot plastics fatigue from repetitive flex cycles and UV rays causing loss in liveliness.
Your liners will wear out much sooner but 200 days of hard skiing in a pair of shells is a long time.
Stronger skiers will notice the fatigue sooner than a more recreational skier but if you could ski a new pair of the same boots after skiing a 200 day old pair, you would notice a marked difference. Darren Rhalves retires his race day boots after only a few races.