The problem is, they deform with the heat from the bit. I have a rub colar from my old router bit drwer that I keep on the 90deg V bit. I had a 1/2" one slide off an expensive 1/2" upcut straight bit into the carve and ruin the bit. The main puropse that I see is the make the first and second insertion of the bit match so you don't get a "refind".
Clint