James at CATS Climbing has an interesting take on grip positions. I agree with most of his conjectures. However, there is room for subtly in this statement - "There is no advantage to climbing without the thumb pinching the bottom of the hold (Grip B)"
If you are climbing in a performance-orientated environment, then there is a role for Grip B. On the other hand, if your inside climbing is training targeted towards outside performance, there is less need for Grip B. It is uncommon to pinch the bottom of edge holds outside. I would advocate spending a portion of training time on thumbless climbing. Yes, it is contrived. Yes, you can't climb as hard. But it transfers better to outside climbing than the the ubiquitous pinch grip found in indoor climbing.
Lastly, I simply don't understand the statement - "None of this applies to route climbers."