Greetings
Damn... Until you made this post I was going to stick up for you... In fairness I don't understand why everyone's taking such a hard stand against this quesiton but... Well whatever...
If you are trying to ask what people without an opinion on a topic can contribute to a debate about an opinion, your question is a tautology by its very nature.
What you have to realise, however, is that a theistic debate is not 2 sided. In fairness, neither you nor I have any certainty of knowing we are right, which as you said, means that we all are technically agnostic.
My bottom line: Opinion is one thing, and fact is very much another. An agnostic is able to have an opinion without claiming to have any facts. That's why they shouldn't shut up (To use your incredibly ignorant choice of words)
GhK.