I think this clarification is very helpful -- in fact I think it would be worth expanding the stock description of "readmore" tags at Writeup Formatting Tips at least a little bit, along the lines you've provided, so newcomers are more likely to get the reasoning for it (and use it properly). Right now, the description is just:

Anything appearing after a <readmore> tag will not appear in a root node unless you're viewing it by itself. This means you can hide long irrelevant pieces of your post (like a code section) by putting <readmore> tags around it: ... (brief example...)

I can imagine some people not quite getting the relevant distinctions from that.