The people behind C&Ds are normally just looking for you to roll over and play dead. If they were into extracting money from you, they'd serve you a suit concurrently.
NickRac is on the money --- to keep a trademark you have to defend it. If an advertiser sends you a C&D, that doesn't always mean they don't want to business with you, its just that you can't do it with the domain, or those keywords, etc.
The thing with C&Ds if they are legit and from a law firm is that it is really the lawyer looking to make money. If you notice the wording in them usually the client has no idea about the infringement and the lawyer is just looking out to get paid in the process.