I have wondered about this a lot, and there are two issues surrounding your question:
1) Does it work? Can you get an accurate reading about a third party?
2) Should you do it? Is it ethical?
I think that the SHOULD part is only really relevant if you think the answer to Question #1 is YES.
If you don't put too much stock in the outcome of a reading on a third party, then who cares if it's ethical? You're essentially doing it to waste time or appease someone. But, I think if you don't really believe in the accuracy of a third party reading, you owe it to a sitter to explain that viewpoint. You might even say, as Gregory pointed out, that unless the question is framed in a way that involves the sitter directly, it's probably not going to mean much. (For example: "What's the status of M's relationship with L?" Is not really the best question. A more accurate response might come from a sitter asking, "What role does M's relationship to L play in keeping M from reuniting with Z?" or something like that.)
If, on the other hand, you do believe the tarot accurately reads other people -- people who are not in the room and who did not ask for the reading -- then I think you need to decide if you would want a tarot reader poking his/her nose into your business without asking.
Ultimately, my feeling about third party stuff is that I can't tell if they're accurate or not because unless you see or talk to the person you're asking about, you'll never really know if you're getting the "correct" response from the cards. I think: take it with a grain of salt.
I also think that oftentimes the cards' answers to questions like, "What's the status of M's relationship with L?" are not really the ANSWER to the question asked, but more like, "I'm going to tell you why you're asking me this question."