This is more of a philosophical/best-practice sort of question rather than a technical problem.

Are there any strong arguments against writing a DataContract class with methods that are to be used server-side only? Or what about additional properties that are not decorated with the DataMember attribute?

For example:

[DataContract]
public class LogEntry
{
  [DataMember]
  public string Message { get; set; }
  [DataMember]
  public string Severity { get; set; }

  public string SomeOtherProperty { get; set; }

  ...

  public void WriteToDatabase()
  {
    ...
  }
}

Not doing it seems like an awful lot of extra work that I would prefer to avoid, although using extension methods could make it easier. But, as a good developer, I am wondering if it is bad practice to do so.