I don't have a problem with the homage part of it but if he fucks it up he'll cop crap for his efforts. If he does a good job people start calling for him to be a permanent replacement until he does fuck it up. It wouldn't worry me greatly I'm not a big enough fan to care who the lead singer is but for the die hard fans it could be a big deal so either way it's a pressured gig.
Obviously the fill in has taken that into account before accepting but look at bands like Queen and INXS who replaced lead singers. The guys who took both roles had massive shoes to fill, I say that without liking INXS, and they haven't been hugely successful because of fan backlash. The guys could sing and did preform well (except when INXS did the reality show to find their singer) but they weren't the original, couldn't fill the original and both bands foundered quiet a bit at different times.