He cut if off because
1) he had enough material for the blade he had in mind (obviously not his first wrench knife)
2) if he didn't, he would have to forge weld the end closed. 10X the work to forge weld, and you always have the specter of a "closed shut" or bad weld that won't hold up
3) The end of the wrench may be too dirty to get clean within a reasonable amount of time.
And again, he used a specific size wrench with a specific sized handle on it to make the knife. His plans didn't include the wrench end.
Robert