You're right, you can't torque the spindle using this tool. The main purpose of it was the other end, the spark puller, and for the hexagonal end... I was thinking it can be used in remote places, where torquing is not that critical, until you would get to a garage or so.
Personally, although I have 2 torque wrenches, never used them :) Also, never broken a screw or stripped a thread, I guess I just feel it when it should stop. In the end, it's 50 Nm, that's 5 kgs on an 1 meter long bar. Using what you have in the bike's tool roll and this socket, and tightening it by feel, should get you pretty close in the 50 Nm zone :)