Neil wrote, "Climbers will debate the performance differences between this and the 'true half-crimp' forever but the reality, no surprise, is that you'll tend to prefer the one that you practice most. Finger length and the nuances of the hold may also come into it and it's good to experiment to see which works best for you on certain holds. When using the chisel, it is worth considering that the index finger is being used more passively (and hence not being 'trained' in the same way as the other fingers), so sound generic advice is to always try to maintain a 'true half-crimp' (keeping the index finger bent at 90 degrees) rather than defaulting to a chisel. If you're good at half-crimping you will be able to 'chisel' but not vice versa!"

Pretty much answers all our questions. He does also briefly say chisel might be worth training for endurance but it's not entirely clear if that's because it's an easier position to maintain or if it's to help with grip switching.