It is sometimes suggested (e.g. by Alison Hills) that there's generally something improper about seeking out or accepting moral testimony, and that to achieve genuine moral understanding we ought to disregard testimonial evidence about moral matters (even assuming we could somehow know the testimony to be reliable). Now, I'm happy to grant that moral understanding is significant in various ways (e.g. in its connection to virtue) that mere knowledge is not. And I agree that testimony will at best yield mere knowledge, not understanding. But, while this may show that relying on testimony is often less than ideal, it doesn't yet seem to establish that it is in any way bad, or that we would do better (in terms of understanding) to disregard reliable moral testimony.
(Acknowledgment: While I made this point in response to a talk at RoME in August, I was reminded to post about it after similar remarks were independently suggested by Cole Mitchell at MadMeta last weekend.)
It's a fairly straightforward point. If I'm currently ignorant of some moral truth, then I lack both knowledge and understanding of it. Acquiring moral knowledge via testimony in no way further reduces my (non-existent) understanding. Perhaps I should (ideally) continue to reflect on the matter until I come to understand why it's true, but previous reliance on testimony is no barrier to that. (If anything, I would expect it to be easier to acquire understanding from a starting point of knowledge than from a starting point of ignorance.)
On the other hand, if I already understand why some moral claim is true, then having the claim further supported by expert testimony should not in any way defeat my existing understanding. It may simply bolster my credence in the claim, and even help maintain my understanding by removing doubts that may otherwise have led me to mistakenly revise my view.
So, whether I already possess the relevant moral understanding or not, accepting reliable moral testimony can be expected to (i) not reduce my understanding, and (ii) possibly even help, indirectly, to bolster my understanding. So, if one thinks that accepting moral testimony is somehow "improper", the value of moral understanding will not explain why.
We should be careful, then, to distinguish two claims:
(1) One generally should not settle for mere moral knowledge, e.g. as acquired by testimony.
(2) One generally should not accept or rely on moral testimony (even as part of an ongoing process of moral inquiry).
The value of moral understanding provides support for claim (1), but not -- so far as I can see -- for (2).