In reply to TimH.

Essentially, yes widgets are just blocks.

However, there is a vital difference between how their HTML is output, which changes a consistent standard that theme authors have relied since WordPress 2.1 (maybe 2.2 -- it's been a while). Taking away that structure means rethinking how sidebars work, or at least rethinking how theme authors approach styling them. There is a definite backward-compatibility issue at play here.

I have some ideas though. New post coming in soon.