In my experience, the single-word last line of a paragraph is generally less objectionable to most typographers than the traditional widowed line at the top of a page. BTW, I've encountered the terms "widow" and "orphan" used inconsistently when speaking of these different situations: a short line alone at the top a page, a single beginning line at the bottom of a page, and a short line at the end of a paragraph. But all deserve a typographer's attention.

Actually, Bringhurst does address your question directly in §2.4.2:

Avoid leaving the stub-end of a hyphenated word, or any word shorter than four letters, as the last line of a paragraph.

But he doesn't offer any additional explication.

What constitutes an unacceptably short last line of a paragraph is indeed often a matter of style. I know book designers with varying degrees of tolerance. Some designers I know will never accept a single word as the last line of a paragraph, but I think this is an impractical stance.

In my opinion, it depends upon the length of the final word, the line-length of the text, and the size of the indent. I will tolerate a single word, but I never accept a single-word last line which is shorter than the paragraph indent and usually prefer the word to be at least double the length of the indent. When possible, I prefer to see the last line of the paragraph be at least 20% of the line length. A shorter line length can accommodate a shorter last line. But that's a loose rule and the (trained) eye is the best judge.

I will accept a single word because often the alternative is less desireable -- overly loose lines to push a word or overly tight ones to take it up. I'd rather see a single-word last line than bad word-spacing (or, heaven forbid, letter-spacing).

-- K.