As much as I hate to admit it, public perception is generally not kindly disposed to such displays these days and 'bearding the lion' and antagonizing those on the fence that may not have had a negative view before doesn't serve the cause.
I don't disagree with the sentiments on the shirts at all, but confine my own wearing of such for trips to the range if I wear them at all.
I've found extreme displays or behavior by either side alienates those on the fence. Like it or not, rightly or wrongly a large part of this battle is going to be won in the court of public opinion. Right now there are expected reports (and I don't believe them) that the "public" wants more gun control. My personal belief is this is the result of 'push polling', or phrasing the questions to get the answers you want. But to non-gun owners, and many sheeple type personalities that, by nature, want to 'blend in' and 'go along with the crowd', even if they really didn't have an opinion before - when asked, if they think that's the way the majority is leaning they'll fall in line.
I choose not to give them an excuse. Just like the way I discreetly carry, conduct myself on the range, and go over and above to be reasonable but firm in my discussions about the issue with those we talking about, I'll reserve those types of shirts for the range/gun club. To each his own.