Let's revisit this discussion from 2008. Has your opinion (or the facts) changed in the last few years?
Many readers are adamant that two spaces after a period or other punctuation is the most readable and most attractive. Voters in the related poll are almost evenly split as to their own preferences. Among those that admit to liking the appearance of one space better there are some who still have trouble conquering old habits -- whether or not they first learned the two spaces practice in a long ago typing class where that was the norm.
Buried in the comments to a Design Observer post about a graphic design exhibition appearing around Europe and the U.S. a single line in a comment by Manual seemed to jump out at me:
Is 'opinion' more important than fact or proof?
[original Design Observer post and comments no longer found]
While Manual's comments accompanied a discussion much deeper than the old one space or two debate, the question still seems appropriate. Despite what studies and years of practice have revealed about how people read and comprehend or relate to varous typographical elements including font choices and punctuation, images, colors, and layouts someone is going to break the rules.
In their opinion it just looks better, works better, or seems more correct than the so-called rules or facts or common practice. As designers and communicators are they being daring and cutting edge, ignorant, progressive, sloppy or lazy, bull-headed, flexible, changing with the times, or what?
And what happens when you throw the opinions of clients or bosses into the mix? Sure, they hired you for your expertise but everyone has an opinion and what do you do when your opinion (or your interpretation of the rules or best practices in design) clashes strongly with the client or boss? Who wins and who loses? What's your opinion on this whole issue?