Do you expect the company to be honest with you if the reason that the previous employee quit was:
- Their manager (your future manager) is an asshole, or micromanages, or is ignorant of technological issues
- They were harassed or discriminated against and have filed a lawsuit against the company
- They were significantly underpaid and left to take a 50% raise at another company
- They burned out from working 80 hour weeks for months on end
- The work environment is in a constant state of chaos, and it's impossible to get work done
- The code base is a buggy pile of crap
At best, you'll get one unsubstantiated version of the story that you can compare against other versions that you hear. Or perhaps you might be able to tell something from the interviewer's reaction to the question. But most probably, you'll get a well-rehearsed non-response like "he was happy here for a long time, but thought it was time to move on".
As I see it, most of these questions are not so much about what you get in the answer but how you get the answer: the metadata you might get at. I also like being direct. It shows confidence and interest. But definitely, I may be letting my naivety get the best of me here.