I've got a few thumbhole stocks on my rifles. Mine are MPI silhouette stocks.
I find that a good thumbhole design puts my shooting hand "in the place it should be" to put it bluntly. Closest ordinary stock that (I find) "puts my hand where it should be" (or close to it) is my HS stock on my sendero, the one with the heavy palm swell.
To me, an ordinary stock grip doesn't position your hand well at all for consistent (comfortable) gripping of the stock when triggering. When shooting with an ordinary stock, I find my hand often positioned halfways "off" (below) the grip altogether! And I don't care for that at all. With (my) thumbhole MPI stocks, I find my whole hand is on that grip, as it should be.. and is easily repeated consistently. They're very comfortable to shoot.. it aids it confidence and consistency of position. To me, comfortable repetition makes for "muscle memory" developing (in good/proper ways) and is very important to ones shooting and accuracy.
I wouldn't trade my thumbholes for gold