Now one of three things is happening here. Either Mát is misremembering (it happens to all of us), he or she is lying, or he or she has made the mistake Daxxari mentions, mistaking the fact that Resilience back then was part of the item budget and thus, PvP gear was less powerful in PvE because it spent itemization points on a stat that reduced your chance to be crit (back then, that's what Resilience did). But no matter how you look at it, the idea of this penalty introduced for wearing PvP gear in PvE did not exist - which is why so many of us wore PvP gear to PvE in. Sure, it had resil on it, but it was easy to get and often better than what we would have gotten from five mans to prepare for raiding Karazhan.
With a game as old as World of Warcraft (we're entering its tenth year) this is understandable. Not all that many people playing today have played since launch, not even since the days of BC or Wrath - heck, there are a great many people who started playing in Cataclysm and even quite a few who started during this expansion. People will tell you that the talent system that we had up until Cataclysm allowed for great customization. They may even believe it.
I once wrote about the pitfalls of players believing that their personal experience of playing the game translates to a universal experience that all players share. That's still a problem, of course, and this one is related to it - we as players often assume that what we remember the game as having been is accurate, that the game was exactly as we believe it was. I do it, you do it. The appeal to authority becomes the I was there argument, and people assert things that are not accurate. Sometimes, we don't even know where these inaccuracies come from. We repeat them in all innocence, having heard them from... someone... in the vast chain of people talking about the game and how it was, and how it is. And the fact that the things we really, truly believe might not be true doesn't even occur to us. In a way, I'm fortunate - WoW Insider has over 179 pages of posts I've made, going back to 2007, so I can see what I was doing and saying about the game. Most players don't have that kind of database to pour over disbelievingly while muttering to themselves about what a noob they were.
I tell a story about the early days of the game. It's a story I love to tell, about the first time I tanked Onyxia. You can see that story in this post. The story that appears in that post is far more restrained than the version I tell now, because memory lies and I've managed (without realizing or intending) to blow it up over the years. I end up half-naked in some versions of the story I tell, not out of malice or an intent to deceive, but simply because that's what memory does.
It therefore behooves us to take not only what other people say about the game, but what we say about it as something to listen to and understand and examine - but never as gospel to be accepted as unvarnished truth. We have to examine what we believe about the game, because whether or not we realize it, our viewpoint is colored by our preconceptions, which are etched by our experiences, and which etch them in return. It's taken me years to realize that my dislike of playing a hunter is based on stereotypes about how the class plays that haven't been relevant in quite some time - the class changed and moved on, and I was still back in BC with the big red kitties of years past. Everything changes, and if you don't let yourself do so, you end up playing in a game that does not resemble the one everyone else is playing.
Just keep it in mind - there was never a mechanic that made PvP gear automatically a hindrance in a dungeon or raid, scaling down the damage and/or healing of those who wore it. I didn't tank Onyxia half naked. Talents did not, in fact, offer greater customization before the Mists revamp. Hunters can equip the Ice Barbed Spear still, but it's basically useless to them now. The game is what it is - it carries what it was around with it like a shell, but those days are past, and we may not even remember them accurately.