I do not mean to sound like the almighty expert on this but.....when my skill level got to the level of really fast (for an old guy), they are a necessary part of me being way off of the bike in the extreme corners. One buttcheek, the balls of your feet and the INSIDE SURFACE of one leatherclad knee are the only points of relationship/control with your motorcycle( I don't include hands because they are busy with controls and should not be part of the balance/weight transfer/body shifting dance). SO...that makes a non-slip surface on the tank very important and a smooth tank or vinyl tank cover is totally inadequate if you rise to high levels of cornering.
BTW, great question, tank grips are not talked about very much. Doyle