IMO and IMHO are necessary
Often, if you say(type) something online that people reading it won't like they'll try to completely discount any validity of your statement by claiming that you're stating your opinion as fact. By adding "IMO" or "IMHO" to what you say, or "I think" (which usually makes it easier to read), you prevent that possibility.
I've noticed this in newsgroups about fandom of one thing or another - when people speculate about what's going to happen in the show/toy line/etc. next, and don't specifically say that it's their opinion or just speculation, it tends to come off as if they're foolishly sure that they "know" what's going to happen next, or are acting like they have inside information. People who write very directly and matter-of-factly often sound like they're stating their opinion as fact, and people who don't have a valid contradiction for someone else's point may use that as a invalid invalidation of the point, so to speak. IMO and IMHO can be valuable to remind people that what they're reading does not profess to be fact, and to prevent people from using cheap contradictions.
Of course, they can be overused. Personally, I don't think it's very good to use the abbreviations in anything longer than a paragraph. It's easier to read if you write out the full phrases, or use alternate terms. However, I think the abbreviations are appropriate and useful for short messages and posts, and do serve a purpose.
You know, "necessary" is really too strong a word. The title should probably be "
IMO and IMHO are useful," but I wrote it right after reading "
IMO and IMHO aren't necessary."
Also, they're only useful in a casual setting. So another better title would be "
it is useful to mention that you're stating your opinion and not fact," but that's really long-winded.