Skip to the bottom "FINAL NOTE" area if you want to avoid reading all of my opinions and skip to the main idea...
Saying "IMHO" and "I think" is almost always unnecessary and redundant. I agree with you that people do it for reasons of being polite.
However, I find that I'm saying that to remind the reader that it is my opinion and I can be wrong. If I state it without explicitly stating "in my opinion" or "I think" I've been told by others that my opinions can be interpreted that I'm stating them as a fact. If I state things as a fact, it is more prone to offend someone.
Thus, whenever I'm saying something that isn't completely positive here online, I will often say "I think" or "in my opinion" to soften my tone and to avoid misinterpretation. In real life, I never say this in person; when people say this, I think it makes their overall statement weaker; I avoid saying this in person (although recently, I've noticed that I've been saying it more often).
"Brand X are great watches." This statement is an opinion.
"Brand X are terrible watches." This statement is an opinion. These are black and white, no misinterpretation here. However, whenever there is something negative, I would always say "in my opinion" to soften the overall statement. "Brand X is a terrible watch company in my opinion."
"Brand X are terribly unreliable watches." This statement can be interpreted as fact or opinion. Statistically, Brand X are considered under par by several standard deviations. However, Brand X is a smaller company with production of only 1,000 watches a year. When taken into context, for a small company, Brand X might be considered fairly reliable, however, to some consumers, they may be "terrible." This has the potential for misinterpretation, maybe not among you, but among others.
Another for instance, I have heard someone say "The Brooks Brothers logo looks like a dead sheep being hung." They're stating this as a fact even though it is an opinion (and a misinterpreted one at that; the sheep is hung with ribbon and this is a historical logo with a deeper meaning about fleece). When one says such a strong statement, it appears that it is being said as a fact when first inspected superficially. However, the person who said this statement really meant it as an opinion by saying "looks like a dead sheep" and didn't say "it is a dead sheep."
All of the above has been stated as fact; but in reality it's only facts to me, it is my interpretation of the facts (which really means it is actually an opinion). And you shouldn't take my word for it, I'm a young person with little worldly experience; I took a "Management Communication" course in college and that's about the limitation of my expertise in this subject.
FINAL STATEMENT: It is prudent to be diplomatic by stating things are your opinion. If you state statements too assertively, your statements might be interpreted as too rigid and as arrogant. I wish I didn't have to say "IMHO" but I find that I do it sometimes.