Grammar enthusiasts provide a great example for how to apply the understanding of the different kinds of 'should's. You probably have seen blog posts along the lines of "X common grammar mistakes" or "X writing mistakes to avoid," or ran into people that correct your grammar, spelling, punctuation, or word choice. In general, the majority of their advice can be abstracted into the principle: "You should follow such-and-such rules when communicating."
Awesome. That's a 'should' statement you should (if you share my goals, at least) love to pick apart. Most of the time, when I hear or read this kind of advice, reasons aren't given other than that the given rules are 'correct.' I think this makes it safe to assume that they believe that their reasons for compliance and encouraging others to comply with the given rules include some reference to an authority of sorts, some nebulous set of the rules of English.
Here would be the opportune time to point out that linguists can generally be divided into two camps, 'descriptivists' and 'prescriptivists.' For prescriptivists, the rules of English prescribe the way it should be used. A speaker should say, "Fred and I" when Fred and she are the subject of the sentence because there's a rule somewhere that demands it. For descriptivists, the rules of English describe the way it is used. For prescriptivists, the rule precedes usage. For descriptivists, usage precedes the rule.
Given my love for constructs and the decontruction thereof, you can probably guess which camp I fall into. (Or "guess into which camp I fall", since we're on the topic :] ) It makes so much more sense to me personally that the rules of English are defined by the usage of the speakers, since it's hard for me to understand what kind of authority could be 'out there' to create the moral obligation to speak English in the 'right' way. A sort of God of English, I suppose, to which anyone that has an accent is a pagan.
So if there's nothing about English that demands the speaker to use it in a certain way, perhaps there is a pragmatic reason that one 'should' follow such-and-such rules?
This seems much more likely, and my personal thought is that the acid test of the practicality of any instance of language usage is whether or not the speaker was understood. I think the common goal behind the 'should' statements of any language rules is "If you want to be understood, ". Once you don't care to be understood, all the rules of English go out the window. Unless you're worried about the God of English smiting you with an Oxford English Dictionary.
Another reason that I commonly hear brought up as for why one should speak in accordance with such-and-such rules is that of professionalism; English speakers that don't speak/write in a certain way, especially in an Internet setting, are deemed unprofessional (and usually, therefore, untrustworthy, which is more important in terms of being understood). I don't like this. I think the perception that a certain way of speaking a language is 'professional' in contrast to other ways of speaking a language is one of the main ways of propagating racism. The way that someone speaks or follows/fails to follow certain language rules has no bearing on her intelligence or ability to perform professionally. Just my two cents.
So, who wants to be a linguistic prescriptivist?