there is a difference between 2% and .0002%... the .0002% would be halfbreeds
there are 6 known... all lore characters or given shout-outs in lore, making a halfbreed of any kind would be a special snowflake because you'd be in that .0002%
out of millions of people canonically(?) there are only 6 half breeds
So, to start with, WoW is a representative world. Not every person living on Azeroth is represented as a character. There are more humans than we see. More night elves. More blood elves. More high elves. More gnomes. More Draenei. And, yes, more half breeds
Secondly, half-breeds in general there's more than six. The six you're referring to are the six lore character half-breeds. This is why you thought of them, and this is why you noticed they were all lore characters. Here's the actual list of every half-breed in the game thus far
-The half-orc/half-ogre beastmaster heroes which were randomly generated in Warcraft III. So, that alone brings us higher than your "six" since it was common enough to be randomly generating names.
-Tagar Bearclaw (who is one of the WC3 randomly generated ones, but I thought he deserved special mention since he also appears in the Horde Player's Guide)
-Garona Halforcen; half-orc/half-Draenei (I know she's one of the ones you were thinking of)
-Lantresor of the Blade; half-orc/half-Draenei (NOT a lore character. Just a quest-giver.)
-Me'dan; orc/Draenei/human/Mary Sue
-Leoroxx; half-orc/half-ogre (NOT a lore character, just a quest giver)
-Arator the Redeemer; half-elf/half-human
-Alodi; half-elf/half-human (though I suppose he's been dead for a very long time so I'm not sure he counts...)
-Lelior; *possibly* 3/4 elf and 1/4 troll
That's quite a few half breeds and they are not all lore characters.
As for whether or not a half-dwarf makes sense. We have a person who is part Draenei, part Orc, and part human. Those races come from three completely different planets. Any argument that a half-dwarf/half-human doesn't make sense or somehow ruins immersion is clearly irrelevant.
Might also be worth mentioning that half-elves are common enough that Kalecgos' mortal appearance is that of a half-elf. The Mok'Nathal are all, or are mostly, half-ogres and we don't know how many of them there are.
And this is all without ever bringing the RPG into this (except with Tagar Bearclaw).
My point, in summary, is that you have NO idea how many half-breeds there are. I'm not even sure you know the lore on them that well. When it comes down to it, there is not enough lore to make that judgment. All we know is half-breeds are uncommon and not always accepted. Anything beyond that is you basically whining that something can't be because there's no specific examples of it in the lore.
But you know what? Before the Burning Crusade there were no examples of beings completely made out of energy. Before Wrath of the Lich King, there were no examples of Death Knights not being part of the Scourge. (I mean, the new Death Knights not the WC2 Death Knights.) Before Cata, there were no examples of traveling *forward* in time from the Caverns of Time. Before Pandaria, there were no Sha.
It's a fantasy world. There is always more than be added. It doesn't need to be added by Blizzard -- that's why we're RPing. To create our own stories and our own lore in Azeroth.
This when you guys come in with the "that's how we get terrible RP" and, y'know what? It isn't. You know how you get terrible RP? By being elitists about RP. By not helping people who are struggling with their RP or their ideas.
Instead of "No, that wouldn't work" try "Interesting. I'm not sure that would work, but let me help you figure it out." Or, "Hey, I think I see what you're trying to do with your character -- can I make a suggestion?" Even "I just want to make sure you're not doing this just to be a special snowflake -- that can be really annoying, and I want to help you make sure the maximum number of people want to RP with you." It opens a dialogue, rather than shutting someone down or making them go off on their own when they may need help. It gives an opportunity for someone who is struggling to improve. This lore-police behavior does not.