I recently had the chance to lurk during a heated debate about the use of the apostrophe-- I know, I know, that may not count as "heated" where you come from, but among some English teachers and writing coaches, these things matter more than the Super Bowl and World Series combined.
I'll add another post or two about apostrophes later; right now, I want to comment on the process that people used to try to argue their point and break the deadlock.
1. Some folks argued that one usage LOOKED BETTER than the other. That's making usage decisions based on aesthetics.
2. Others said that when they googled a certain word, MOST PEOPLE did it a certain way. That's making usage decisions based on consensus.
3. Then there were those who appealed to their fourth grade teacher, their best friend's first cousin, or some HANDBOOK or style sheet they dug up somewhere. That's making usage decisions based on authority.
4. Finally, there were a few stubborn stalwarts who insisted that whatever THEY HAD BEEN DOING for the last upteen hundred years or so had to be right because, after all, that's what they'd always done. That's making usage decisions based on habit.
Aesthetics? Consensus? Authority? Habit? When we are not sure what is correct when it comes to matters of punctuation or usage, what should we do? Or, more to the point in this post, what guiding principle do we use to make the decision?
5. The debate was broken when someone (dear old "anonymous") pointed out that what is correct depends entirely on what MEANING you are trying to convey. That's right. Punctuation, like other matters of usage, is intended first and foremost as a servant of meaning.
In short, it doesn't make any sense to ask whether it is better to say students' or student's: the question is, how many students do you mean?
Commas, semi-colons, periods, all that stuff: it doesn't have to do with needing to take a breath, to look good on the page, to pay attention to Ms. Turabian, to sound right, to fit in the the crowd, or do it again the way you've been doing it for time out of mind. The first thing you gotta know is exactly what you are trying to say. Then you do your homework and get the best information possible to help you say exactly that.