The end reason for why this bothers people is because "unique" makes games "fresh", and for far longer.
Street Fighter 2 is an old game, it was fine back then with console limitations, but now the cast is far larger. It's a poor example, as is WoW. Wow also recieved flack for it's lack of classes as well, but that was until people found out that the talent trees gave you a lot of customization and each class already recieved like 40+ abilities, compared to other games (like FFXI) where you received at max 10 and without customization.
DoTA does have a lot of heroes that aren't used and a small elite cluster that the skilled players stick with, but the reasoning is simple: Balance. That's still also a poor excuse for lack of heroes, even though DoTA can't balance their heroes well (and I don't blame them, balance is very hard to do in many games because you have to predict so many situations..) they do have a roster of very unique and different heroes. Not bad for a mod.
I'm not attacking Demigod or the Dev's, I'm sure there are a lot of reasons why we only have 8 Demigods at the moment. Still, I agree with the OP. It's very unfortunate that we don't have at least six more at release to mix things up a bit. I'm sure more will come eventually and I'll be there on launch day regardless, but it still would of been nice.