A pet peeve of mine.
Everytime I go to paste some code in, I always pick Code Span instead of Code Block, then have to fix it up. Lots of people don't realise they've made the mistake hit post and we're stuck with the ugly formatting that is not only visually loud, but takes up more vertical space and is hard to read.
I could train myself to use the shortcut, but training everyone else to do it is much harder.
Personally, I think the Code Span is completely pointless - The idea of writing code in the middle of a line seems insane. So I'd prefer the button was just removed completely.
However, I recognise you're unlikely to do that, so my compromise is if someone selects multiple lines, and chooses Code Span - just use Code Block - I guarantee you it's what everybody always means.