I've seen it in multiple places, but it's mostly them just dissing Willow and Oz's relationship (that I am a fan of too) that kind of gets to me. They're so quick to dismiss that relationship but raise hell when someone dismisses Tillow. Plus they seem to say they were soul mates or the perfect relationship and they had so much in common, but I found that the Tillow relationship wasn't like any of that and I liked it despite that.
Well, what did you expect from hardcore shippers? 🙂 Personally I love Tillow but dissing other ships or characters in order to "prove" the awesomeness of your OTP seems very silly to me. I mean, even if Willow/Oz is somehow "proven" to be the worst relationship in the history of the universe this wouldn't make Willow/Tara any better in itself, would it? Such attitude can be extremely annoying in fanfiction - many a writer prop up their favourite pairing by making characters or pairing they dislike look like fools. Heck, I would argue the show itself did it when they destroyed the Scooby friendship in S6 so that Buffy would have no choice to turn to Spike for comfort...