I actually like the idea. Its NOT blind firing, because you DO have a line of sight. You would know if somebody is at point blank range or far away. Its the same concept that surrounds the corner shot.
I don't think people would take offense when you explain it like that, they would probably call it cheap because the user would not have to expose himself in an exchange. In a regular face to face exchange, at least both sides have a fair chance to hit the other, with this I think people are calling it cheap because it gives the user the ability to fire and not worry about retaliation.
Then people would think that guns with a longer range than theirs are cheap, because they cant reach the shooter but they can be shot. Everything has its advantages and disadvantages.
That's a good example, but not exactly in line with the reality, most engagement distances will not be at the maximum range of even a well tuned airsoft gun, they're usually much closer for the simple fact that as madsarge said, most people can't judge distance. Let's face it, if all airsoft engagements were at the maximum distance of well tuned airsoft guns, that's more reason for cheaters or shady players to not call their hits, the more distance you have, the more doubt can be raised, after all, can you see a 6mm hit a 6'0 target at over 300 feet away? Also take into consideration that at maximum range if that BB hits gear it'll probably hit with the energy of a baby fart, you could be an honest player and not even feel that if you weren't paying attention.
I tend to think in terms of worst case scenarios, and besides the one I mentioned where a straight shot hits the lens at nearly point blank range, I think there's also the potential for cheating with this. Gun hits could be cast into doubt with sufficient distance and the simple fact that someone using this could easily say, I didn't see or feel the hit on their gun, then in turn not call it.
From a practicality standpoint, there might be a handful of people here who could be dedicated enough to practice with this enough to be proficient, but lets not kid ourselves here, 99% of the people here would never really use this to its fullest potential. Besides if you really were a LEO/operator, the real solution would be to use
CornerShot :p
For the money, I think a better investment would be more Thunder Bs, its not like the walls in a CQB arena are thick, you can hear people on the other side, and its way easier to just throw a Thunder B into their room than it is to try to engage someone with an attachment that you probably won't use proficiently.
