Odd. In the newsgroups I frequent, it is often used after a statement, rather than before it. The context in which read, and use, it most often is in discussing the merits of a particular piece of software, appending, or prefixing, IM(H)O to stress that the statement is subjective, not necesssarily objective. It is a useful tool for preventing flame wars from those who disagree with the HO expressed.
I've seen it in newsgroups a lot, but moreso on chats.
And I'm not annoyed at all by "IMO," but only IMHO. It's not something that gets me angry or disgusts me or makes me grit my teeth. It's just something I find mildly annoying, nothing more. Each of us has our peeves and this is one of mine. There are phrases that really do come close to irritating me such as "Take my word for it" or "You don't want to know" or any of various other presumptuous phrases.
But back to IMHO, I've seen it used in the way you've described. And I've seen it used in other ways. Often, it's kinda like emoticons (which I never use, but which generally don't irritate me). Some people think they can say whatever obnoxious thing they want so long as they put a smiley after it. Another example is when people preface a statement with, "No offense, but <insert really obnoxious personal comment here>," or the one I hear from the occasional neighbor, "I don't mean to intrude, but ..."
I'm not opposed to people using language that ameliorates the potential for slight, or acknowledges the awareness one has for one's own fallibility. I think these are great things to do when they are sincere, as they often are.
Ideally, one would never feel compelled to do this. Each of us would practice comity and would assume that others did as well. But I understand in a world where people often speak in extremes, are eager to give offense and eager to take it, that a disclaimer could seem almost compulsory.
k
