When you put it that way it does sound dumb.. the same goes for people that say "like" all the time, even if not to a valley girl extent. It's almost a tick for people, once they start doing it, it becomes a crutch or an "out" for not using proper grammar, because frankly a lot of people don't care about grammar because they either found it difficult or inane. I'm guilty of using "hehe" or "haha", but I try to keep it relevant if and when I do. I suppose you could say it's interchangeable with lol, both in meaning and usage.
There's something to be said for being understood and accepted too. People with more than 15 IQ points separation will have trouble communicating, and it just gets worse the larger the gap. The same could be said for language, someone with a huge vocabulary and the ability to use it might seem pretentious to his/her peers, and so they dumb it down a bit for daily use. As far as acceptance, evolutionarily speaking, the weaker or different ones are singled out and destroyed (survival of the fittest and all that). Obviously it's not quite so drastic in modern western culture (for the most part), but you can imagine why it might be beneficial at times.
Then there are just people who don't have the vocabulary or don't care enough, those people are either misrepresenting themselves or aren't able to properly communicate in written form. Or they're just young.. as you said, that explains some of it too.
Don't put "lol" in your PhD thesis.. but it's fine an email to a friend. Tailor the text for the occasion. If you want someone's respect, earn it. (In other words I agree, lol!)
