This is not necessary. Non-unique tabindexes are perfectly legal: the document order is used to break ties.
I'll have to eat my words here: apparently this is necessary to make r62139 and friends work.
And I'll have to eat *that* as well: it *is* necessary to set tabindex=1 on both wpTextbox1 and wpSummary, because they don't always appear in the same order (wpSummary appears before wpTextbox1 on section=new). Reverted in r62249.