My wife and I play both seperately and together, and we always at least drop a polite 'no' email to pretty much everyone who emails (unless they are just totally rude/offensive or have not read at all our adverts). We think it just polite to do so. People often mail back amazed that we have taken the trouble, and sometimes it has allowed us to open a dialogue with someone we have originally rejected. We have met a few people and had very good times with people we have originally rejected but have sent a nice polite mail to thank us for taking the trouble to write back.
As J is away for a couple of weeks, I've responded to a few ads - politely, ensuring i have met the ad's requiremnts (i know how annoying it is when people don't respond in the way you ask!) and have not had a single response!
Why not just take a few moments to just fire off a standard reply at worst? Politeness does not hurt or cost money!
There... I feel better now - rant over