How a player holds the bat while in their stance is a style choice and not part of technique. As long as the player can get to the bat lag position correctly and on time, it dosen't matter how they start the bat. The advantage of starting the bat out on plane is that the back elbow and front forearm are pre-set very close to where they need to be when they get to the bat lag position. The same can be said for players who start the bat at a 45 degree angle across the pony tail hole. The difference is a matter of degrees. In both cases the coaches and instructors are starting the players out with flatter handsets because they know that the hands need to be flat very early in the swing in order to get on plane quickly and hit in a long zone.
I see a lot of FP hitters who start with flat handsets, including ones who start with the bat at a 45 degree angle across the pony tail hole; drag the bat. IMO, the players need to be taught how to correctly get the hands flat in such a way that they can do it from any handset.
If a player has not yet developed a style would you recommend this? My problem with it is that IMO it does not create enough bat speed, it seems slow to adjust to all the different pitch locations and does not feel powerful.