To be honest, I'm not a big fan of those exclusive items - imho they ruin the item fever which is an essential component of the gameplay in a rpg like DKS. If the exclusive items are so cool, why would you bother searching for cool gear ? But if the exclusive item isn't cool, why would you bother getting them ? And if you use those super items, then you're probably going to be tempted less to explore the world, and focus more on the main quest, which will lead to you missing one of bigger strengths of the game - the fact that there's plenty of stuff to do all over the place and so forth... it essentially ruins the balancing.
So personally I really think that the best thing you do with those exclusive items is sell them to the first merchant you see so you're not going to be tempted to use them and donate the gold you get to charity, but that's just me of course

The other reason is the word "exclusive" - various vendors want something exclusive in return for promoting the game, so you have to make sure that it's exclusive, which is why we can't offer it (and given my thoughts about them, probably wouldn't) . I actually once proposed a designer commented version of the game to a vendor (basically each part of the game would have commented notes by the designers of the game, what their vision was, what it turned out to be, some secrets etc...), which I thought was vastly more interesting than an exclusive item, but the response I got was - that's very interesting, but can you then also add in an exclusive item or 10 ?
So in short, and I think this goes for the majority of developers, if you buy directly from a developer, you support that developer as he gets more of the income than he usually would get , but you can indeed miss out on some promotions which the developers can't make available themselves for various commercial reasons, the word exclusivity being one of them. You can compare it to buying eggs directly from the farm - you probably won't get the cool toy that goes with it if you buy it in a supermarket either
