I think it's highly likely that the blockquote button is mostly for those wysiwyg users who don't want to concern themselves with html or css. If you're already knee-deep in specialized css for specific xhtml elements, how handy could clicking the blockquote button really be?
I think there's a bit of a disconnect here between user-types. "Under the hood" type ePub creators are probably not going to love the code that's autogenerated by features that are mainly intended for users who don't want to see the code behind their work.
It's unlikely that Sigil will parse all CSS (inline and external) to see if there's any styling in place for the element that one of those buttons is about to create. There's '+' and '~' selectors (among other styling scenarios) involved that would make it extremely difficult to account for it all programmatically. So the button creates what it has to to allow the wysiwyg user to increase/decrease the indent without messing with code. Those who are comfortable in code view will either fix what the button creates... or not use the button.
I could be all wet, too. That's not outside the realm of possibility.