You know, this situation brings up an interesting question which is something I have been wondering lately.
Do you think that many companies use mystery shoppers simply as a marketing ploy - i.e. rope in new customers - the mystery shoppers!
Some shops have requirements that make me suspicious about this. For example, I have done shops for one company that shops Skechers shoes and one of their requirements is that you "name three things you learned about Skechers shoes". Okay, well, if the sales person TOLD me some things that I can report, sure. But what if they didn't? They want me to go out of my way to "learn" about the shoes? Why? So I'll become enamored with them and buy them? Or tell my friends about them?
At any rate, regarding the issue of having to give out email addresses, I agree with the others who recommended having a separate email for junk mail. I have one "commercial" email account that I use anytime a situation like this arises, and for any situation where I purchase something, or a company wants to put me on my email list. That way, I keep all the commercial and junk mail out of my regular email box and I don't mind giving this email address out.
You can get free email accounts from Yahoo or GMail.