Here's my opinion...
This debate is all about where to draw the line. Piercing a fish certainly isn't as bad for it as fishing, but being as we approve of that, are we being hypocritical when we say no?
Personally, I'm all for survival fishing. It's completely natural, being as a man's gotta eat. On the other hand, I'm totally against sport fishing. Sure, it does bring attention to aquatic conservation, but don't public aquariums do that? In addition, it can't really help that much: hunting for sport does nothing for zoology, as native animals (and fish) are not perceived to require much protection.
With that in mind, I am against piercing fish. Not only does it take away from the natural beauty--the metal itself is incredibly distracting--but it gives the fish pain, or at least an uncomfortable pressure point. (In my opinion, different fish have different levels of pain competency, but that's irrelevant.)