Photographers are trained to be critical of the images they see, either through self-study, course work or through their vocation. That is the essence of being a good photographer. One must have the ability to see an image and, with razor-like vision, cut it down to its component parts and analyze them in order to produce a good photograph.
Some people develop this habit automatically. For the very best, it becomes second nature.
I can not walk into a movie theater and look at the screen without seeing what's right and what's wrong with the picture. Sometimes, I get so bad that my wife refuses to go to the movie theater with me. One time, we went to a movie theater in Charleston, SC, several hundred miles away from home and, when the movie started, the projector wasn't framed correctly, causing a frame line to blink on the edge of the screen. My eyes were glued to it. I couldn't watch the movie. At first, I went out to the lobby and complained but the problem didn't get fixed. I went out to the lobby again but nobody was there. I looked around and found the door that led upstairs to the projection booth. I peeked upstairs but nobody was there. (The projector was running totally unattended!

) So, I sneaked upstairs, fixed the problem and went back down to the theater to watch the rest of the movie. This was a theater that I didn't work at and had never visited before.
Yes, I know it was probably an a$$hole thing to do but I saw a problem, tried to get somebody to fix it and didn't get satisfaction. First, I am a professional theater technician. What was I supposed to do? Let a theater full of paying customers watch a crappy picture because some doofus was too busy chatting up the chick at the concession stand to do his job? This problem took less than 15 seconds to fix. Literally a turn of a knob.
Developing a critical eye is one thing. Politeness is another thing. Yes, it is sometimes better not to say anything.
However, consider this: The guy who gives his unsolicited opinion cares enough about photography to give you his opinion on your photo.
Okay, it was only an advertisement. The guy was probably off the deep end.
If one is going to give unsolicited critique, I would suggest he be diplomatic about it and give a proper, balanced critique and not just slam the other guy's work.
No matter how you slice it, to display artwork in any form, by it's nature, invites criticism. It has from the beginning of time:
[video=youtube;W_v_ubcYsTI]http://www.youtube.com/watch?v=W_v_ubcYsTI[/video]