For what its worth here is why I disagree:

1. I support more end users than any other Adagio dealer
2. We ask our clients not to use Microsoft Security Essentials or other free and/or inexpensive AV solutions
3. We have had one client with this problem. They are using Microsoft Security Essentials against our advise.

You have to ask these questions:

1. Would you prefer to spend money on a proper AV solution or spend money to solve the problems caused by a free AV solution?
2. Would you prefer that Adagio spend money so that you can use a free AV solution or would you prefer that Adagio spend money improving the features in Adagio.

You could substitute 'inexpensive' for the word free as the AV solutions that we have the most problems with are the ones that are free (Avast & Microsoft) or inexpensive. Kaspersky was a huge pain until be figure out how it needs to be configured (and then we almost never see it), McAfee and Sophos we almost never see so I have no experience, and the managed version of Trend Micro doesn't seem to cause any issues when installed and configured correctly.

We recommend Symantec Endpoint Protection (a managed solution), we use this solution internally, and many of our clients use this solution as well. Yes it costs money to buy and more money every year for maintenance (as its important to keep the AV software current) but we never have a problem with it.