Why the preference for "em" and "strong" over the intuitive and already-used-in-wp "b" and "i"?

Lack of a closing element already wonks up pages, so what will XHTML do? Nullify the unclosed element? I would hope they could make the system smart enough to know (e.g. by programming it with knowledge of the exceptions) when items are stand-alone elements so one doesn't have to do extra coding to indicate that.