radio astronomy - Has SETI data been used for astrophysics? How, or why not?

Well, that might depend on exactly what definitions you use. SETI itself is a broad program. In this answer I'll focus solely on what I consider the most iconic part of modern SETI, which is SETI@home.

We have the following quote from SETI@home's webpage (emphasis their own):

SETI (Search for Extraterrestrial Intelligence) is a scientific area whose goal is to detect intelligent life outside Earth. One approach, known as radio SETI, uses radio telescopes to listen for narrow-bandwidth radio signals from space. Such signals are not known to occur naturally, so a detection would provide evidence of extraterrestrial technology.

SETI@home's specific purpose deals with the radio SETI data. So they are implicitly asserting that the data they collect is expected to be useless outside of their namesake: they're specifically looking for things that are not known to occur naturally.

That being said, it turns out that the SETI@home data collection is almost never active (see here on their site). It is done passively, as a piggyback collection when the Arecibo radio telescope is being used for other science projects. So if your question is "is any data useful outside of SETI actually collected while SETI(@home) is gathering data?", then the answer is "yes".

Now, we can also note that one of the original goals of the SETI@home project was to prove the viability and practicality of distributed computing for scientific projects. This we can say was a resounding success, as it spawned BOINC, which serves as a distributed computing platform for a wide range of non-SETI science projects: Folding@home and Rosetta@home deal with protein folding, Einstein@home is for gravity wave detection, the now-defunct Mesenne@home was for finding Mersenne primes, and many others (my copy of BOINC specifically states that there are over 30 currently active).

So if you are willing to consider the advances they spurred in distributed computing in scientific projects, and how this has impacted scientific projects inside astrophysics (or outside), then the answer is again "yes".

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EDIT: Einstein@Home recently made the following announcement about a gamma ray pulsar that was detected via the program; it includes links to a published article on the detection, including an arxiv version link if you find the official publication is behind a paywall. So that's a concrete, published, and current discovery in astrophysics. If you're willing to give SETI some of the credit for having spawned the @Home style of distributed computing projects in science, as suggested above, then there you go.