Maybe I'm just an old-timer, but I actually like using plain HTML markup for posting here. To me, it's simpler than having to learn another set of rules for each site. (I don't post to a lot of other sites, and I admit I find it difficult to remember which one uses what.)
The square bracket notation is very handy, but those create links, not formatting changes, so I don't have a problem with those -- they're orthogonal to markup, so to speak.
Not that I have a problem with your suggestion; I'm just stating my preferences.