I completely agree with both of you on this. Terminology is not always on the mark for everything.
Other than the case that USMCBIPOLAR mentioned, I prefer the word "recurrence" for when mental illness symptoms return. I also often say "breakthrough symptoms" or "breakthrough episodes", especially in the cases where one is being treated and actively working to prevent new episodes.
sarahsweets, I am glad that you did mention the serious issue of how mental instability does make some people more vulnerable to drug and/or alcohol abuse relapse. That was a major issue for me, at times, during the years I spent in alcohol recovery. Luckily, it hasn't been an issue for me for quite a while, though. The reasons why are another topic.