Two things that strike me about the usage.
First, given that the two branches falling every six times six years (in one account) were supposed to be found by chance, the expression probably contained some amount of self-congratulation: rather than waiting or happening upon the lucky branches, the scholar is said to have gone to the moon and broke them off by his own effort.
Second, given the story of Wu Gang (which was current by the time this chengyu seems to have been coined), there may also be some measure of ironic complaint or self-mockery. After all, the guy who tended this tree and broke off branches all the time got nothing for his effort but endlessly more work.
Haven't seen any of the sources for the phrase mention that, though, so just noting it here. — LlywelynII 13:47, 13 November 2013 (UTC)