Hi Abi, sorry for the delay in coming back to you. Still playing a bit of catch up after taking leave over Easter!
Yes, our formative quizzes in the Moodle format are marked and feedback is given, usually to both incorrect and correct answers. I agree, I think this helps students to use them as a benchmark for their own progress and encourages their use. I suspect if we did not give them marks or feedback, their use would probably drop significantly.
Given that feedback comment is hopefully not that time consuming to prepare and the Moodle system seems to deliver a marking system that works and is easy to understand, we do seem to find these resources quite effective.