In a marketing and publisher point of view what Tynt is providing seems very beneficial for their needs. On a UX point of view it's a double edged sword in my meaning.
You could argue that the user experience benefits from the auto generated addition to copied content since it seamlessly enables the receiver or "copier" of the content to keep track of its source. Thereby providing a shortcut that the user doesn't even have to initiate.
However, one could also argue that it is dangerously close to breaking the seventh Schneiderman rule of interface design, Support internal locus of control. I mean this because if there's no way to disable the feature, the system will format copied content in a way that may not meet the requirements of the user.
This is a hard question to get give a definite answer to, at least I find it so. It would be interesting to hear what other users have to say in this matter.