Forstner bits are very useful in the shop and in timberframing. The problem with using them in timberframing is they are not long enough for the mortise. Bit extensions are the easy and affordable way around the problem. These extensions solve one of the major problems with Forstner Bits---their limited depth capacity. Each extension is 4" long and is sized for the appropriate bit shank diameter. Bits are securely held in place with two hex screws to hold the original bit shank.
I find both the 4" and the 10" are extremely handy.