Right, but they are essentially cannon-fodder. They cannot fight as well as the good soldiers so you're wasting bodies... which would then need to be filled again, and with recruitment so bad you NEED to take thumbless soldiers I think you would hinder yourself more in the long run then helping in the short run. Besides, you can say it's a "cusiher job" but they could also be doing all the manual labor. Built the camps, repair walls etc, while the good soldiers avoid it, also, you need those good soldiers up front so besides manual labor, logistics is the only logical place I can see for the thumbless ones to be placed while actually getting some good work out of them.
Was recruitment really so bad that they NEEDED thumbless soldiers, or was drafting them more as of a lesson for others, that cutting off your thumb is dumb because you still go to war. Drafting them would certainly send a message to others.