When you are researching a topic, before asking a question on SO, you check on whether that question has already been asked (even indirectly). If not, then you ask.
After asking and waiting, what if you do not get a relevant answer? On reading responses/answers to related/relevant questions, you might think "this guy would probably be able to answer the question better/best." But he has not answered or even commented on the question. Maybe he has been busy (in which case you cannot do anything), but maybe — since there are so many questions on SO — your question skipped his eye.
There should be a way in which we can ping or poke "that guy," to tell him about the question.
There is a spam fear factor, when you poke a guy again and again, or poke for irrelevant questions.
The team can come up with a mechanism to prevent people from spamming others with irrelevant questions, but I think if the other person feels disturbed/spammed, he can vote the question down, ask for it to be closed, with his comments specifying the reason, and a moderator or even the community can take care of it with five votes to close. I think.