Used properly, it can actually be affectionate. Commenttards is affectionate and not derogatory at all. When talking about "teh piratzeses" I have no problems with the usage of "freetards." Even if you are talking about people who stubbornly only use free (as in beer) software. Using freetard to talk about people who are advocates of, or staunchly only use “free as in beer” damned near anything seems perfectly acceptable and not necessarily derogatory to me.
I think the line gets a lot greyer when you start talking about "open source advocates or users" as freetards however. It betrays a marked lack of understanding about open source. First of all, open source isn't free. The GPL sure as hell isn’t free. The GPL is a viral licensing straightjacket that exacts its price in controlling derivative works. Other licenses are “free as in beer” for personal use, but not commercial. Some rare few are “free as in speech” and “free as in beer.”
As open source is not about being “free as in beer” but rather “free as in speech” I think that using the same term (freetards) for both groups of people is the mistake that gets everyone’s Irish up. While membership in the two groups does overlap, the crossover is generally less than most people believe.
The proper term usage should probably be:
“Free as in beer” advocates: freetards
“Free as in speech” advocates; opentards
That said, appending “tard” to the back of a word doesn’t really make the intent offensive. I am sure that there are some cerebrally less-capable individuals who would take offence to the entering of this term into the common vernacular in this way, and frankly they probably have a damned good point.
The fact remains that it has been used as a suffix widely and for a long enough time, not only on the internet, but in the real world in non-offensive contexts that it can’t always be taken as some form of attack.