as written before: it's not about standards compliance, but about avoiding you a big deception if scribus applies a fake bold or italic, you don't notice it (for any reason) and you get back from your printer hundreds of printed copies of your book with unreadable (or hard to read) italic.
manually choosing a font variant is typical (or at least was typical) for layout programs, that's probably the main reason why scribus still does not have a B and I button.
but, as you say, there is a need for such a button. it should not apply fake bolds and italics, but have an heuristic to find out if the current font has a bold or italic variant (and most do have!).
and possibly offer a way to manually define such a relationship, too.
if you just want to do it in a best effort way, for the most common cases, it's rather simple to implement (if i'm not mistaking i have code that does the contrary: find out if a given text in scribus is italic or bold).
if you implement it right (and i can give you hints on what could be a good way to implement it), i'm sure that the team would be very happy to accept patches for it!
it's not a solution for right now. sadly, such a solution does not exist... right now.