I think the answer is "because they can" and "because intel cant" (using their current architecture in which a quad core is simply 2 dual cores on the same chip) ... but I could be wrong.
And like you say, it may provide a better cost/benefit option than dual or quad for some applications. Thereby giving them a small section of the market where they are sole supplier with no competition. Of course, this doesn't mean they can charge the earth for triple core, because that would simply drive people to quad or dual core instead.