This mod would be easy enough to add to the
Magical Text Formatter (which is always more up-to-date than the public
Text Formatter), but it begs the question of usefulness. Why do people use the longer STRONG and EM tags instead of B and I? Because logical styles are more
CSS-friendly than literal ones -- you can stylesheet up STRONG to do anything you want, i.e. italics and larger font with no bold. You can do the same with B, but it's less "logical" -- a bold tag should embolden, and nothing else.
However, what reason could there be to employ STRONG but still I, or EM with B, on this site? STRONG is still rendered as bold on E2's stylesheet, and EM is always rendered in italics. Choosing between one or the other is a choice between logical and literal styles, but it has no effect on how the writeup is presented to readers. That being the case, you may as well choose between all-logical or all-literal. Choosing both is worse than choosing "wrong."
Combining STRONG and EM into the same checkbox is just a space-saving measure in a document which is already longer than I'd like, even though it's no longer than is necessary. If other Edevers think this is worth adding, then it'll certainly be easy for me to add. But without a little more support, and knowing that the vast majority of E2 users prefer the shorter literal tags anyhow, I feel it's a lesser evil to leave the two options combined into one.