[axisK]

I'm not sure that specifying how things won't work is done to appear superior rather than giving feedback. There probably isn't enough time in a day to try everything and if you can save some time by pointing out why something won't work off the bat it's probably a good thing. As with everything though, it kind of comes down to how you communicate that something won't work.

[_ea1k]

It also comes down to being right about it. It's often much easier to write something off ("that won't work"), than to accurately assess the reasons why it may not work, and possibly be part of modifying it so that it will. That difference is also subtle (just as subtle as the presentation in many cases), but also very important.