I think I understand your point and I’ve also pondered quite a bit on whether the effort required to maintain an MOC is worth it. I’ve realised that for many cases it actually makes sense if you see an MOC as either a project or a perspective (like in Omnifocus).
From Nick Milo’s example vault, the most interesting one I found was the Quotes MOC and all it had was a list of tags ( #onLife, #onLearning, etc.) categorised by major topics. And that idea changed my view of an MOC totally. I like saving a lot of quotes and I’ve been trying to restrict myself to use only up to 3 tags per quote, otherwise the tags go unwieldily. Even with that, I end up having a lot of tags and this is not even considering the tags used in other types of notes. With the inability to manage tags like organise them hierarchically with groups that we can fold/unfold and search with multiple tags, it becomes tedious to even keep track of all the tags we have from just the tags pane. With a MOC, however, two things happen without much effort,
- Whenever adding a new quote, we can quickly refer to the Quotes MOC for an existing list of all tags used in quotes before. This reduces the possibility of creating tags with similar meanings like say #onLife and #onLiving.
- This becomes an entry point now whenever we are looking to find a particular quote or just in general some quote about certain topics.
Can those 2 things be achieved without MOCs? Sure. For #1, someone had mentioned that they keep a list of all tags they use and always refer to that to avoid synonymous tagging but that sounds like a TOC or MOC itself with the same issue as the tag pane though that it represents all the tags in the system vs the tags for only quotes. For #2, one solution could be prepending tags like #q_onLife, #q_onLove, etc. might solve the absence of tag hierarchy but seems less clean to look at. So I think MOCs solve this in a decent and clean way.
Besides the specific use case for Quotes, I think even generally using MOCs could provide a perspective for what I’m working on right now. In your example of Cats, sure you could tag everything as a #cat and maintain no central list of cats. The problem with that is the lack of filtration. I might have mentioned #cat in both notes about cats themselves but also in my daily notes or some other place where I saved a cat video. While searching then, I cannot find anyway to filter out those entries whereas with a MOC, that is already taken care of. Is a search faster than generating the MOC? Sure. But that one time work saves up many searches (especially complex ones with boolean operators) in the future.
Also, if anyone comments why even save cat gifs in some notes if you want to filter them in searches about #cats, well, if we can’t save cat gifs in our notes, then what’s the point even of taking digital notes? 
