You can get more extension through the zone if you let the top hand slide off when the top arm can go no farther without folding.
I don't necessarily teach releasing the top hand but I don't try to change it, either, if they are releasing. The critical part is that they do not release until after contact.
When performing the Lau one-hand drill (the one that recent posters are calling the reverse hand drill) it is pretty obvious that the ball can be hit harder when the bat is unrestricted by the top hand. It is more than just style. Doing it correctly while batting can definitely be an advantage.
But since no two coaches seem to agree or understand it it is probably best left alone. It can be a great advantage, though, for those willing to learn it.
Most of the coaching I have done has involved 2 hands on bat at all times. Recently, my DD has been moving more towards letting go with the top hand a couple frames after contact. What I have been noticing in the videos is she seems to have more bat speed at contact (based on bat blur), and more importantly, IMO she has a better hand path thru contact. She finishes higher instead of having a flat swing. She used to roll her wrists right after contact which seemed to reduce her bat speed at contact.
Again, not trying to dictate 1 style or the other but letting go seemed to help DD get past one of her problems.