Q: What practical reasons are there not to set
A recently-revived thread on recognizing sentences that don't end in two spaces inspired this question.
The Emacs manual node on sentences notes that the sentence commands assume that we use the American typist's convention of putting two spaces at the end of a sentence (as opposed to, say, the French convention of one space). Arguments for/against the two-space convention apparently gets pretty polemical (eg, this Slate piece).
I don't particularly care about looks insofar as I let LaTeX handle my type-setting, but the two-space convention is pounded into muscle memory. However, when I paste text into an Emacs buffer from other sources (webpages, etc.), it's almost always in the one-space convention. That's kind of annoying in a how-dare-they-differ-from-me sort of way, and I'm tempted to set
nil to Deal With Reality. I never have, however, because I've always assumed that Something Will Break if I did so. I just don't have any firm basis for that assumption.
So: are there any practical consequences in terms of loss of functionality for setting
nil? The only significant change I can think of is that the fill functions will change the two-space convention into the one-space convention.