what information are you gaining by looking in the bore? When would you need to use this? If the gun shoots well does it matter? Just curious why I would need one. I have thought about buying one before but didn't feel it was justified.
I was told a while back (actually years ago) you had to have one in order to line up low profile gas blocks on an AR; but now I am hearing there are other ways to do this. So strictly speaking we don't NEED one. I think they could be handy for preventative maintenance; maybe for spotting erosion around the gas port on an AR; but this would only be an issue if you have a lot of rounds through the barrel.
I just got one and for the money it is surprisingly effective.