Right Thumb....
 

What is the purpose of the right thumb in the swing?

IMHO not much-but it's very handy for teeing the ball up.:)

It can act as a reverse pressure point on the backstroke to feel the 'drag' of a lagging clubhead takeaway....

Ye olde dragback is thought of as something from the past. Whilst it does have historical value from golf teachings from ye masters of ye past, it still remains a very viable procedure. I would suggest keeping it controlled however by never going beyond the flat condition with the right wrist as it was per standard address. Maintain the extensor action also - don't let your stroke become 'sloppy'.

It's an option.