Sorry bit slow on a response - too busy changing nappies!

I use the rear brake everywhere on a 1000, never used it at all on a 600. It settles the suspension before braking hard into a turn, helps you keep a tight line in a corner, particularly if you have gone in too hot and then keeps the front down on the way out. If you watch someone like Glenn or Alex when they have a rear brake light it is on way before the corner and stays on until well out of the corner, they are just using varying amounts of pressure. In tight right handers like turn 1 at Wakey or turn 9 at EC you need to use the side of your foot which is difficult, faster right handers are fine. I've never thought about a thumb rear brake as I use the rear without thinking about it anymore but it would defo be easier to reach etc, my concern would be whether you still got good feel for the amount of pressure you are applying.
As someone mentioned though not all pro's use the rear brake so maybe it doesn't necessarily make you quicker, probably just smoother. For someone like me who has never done motocross/dirt track etc I don't like it when it feels out of control so it probably gives me more confidence.