Like with the word hey.. I don't just flat out say... "hey". In person. I'll usually be more like Heyy! It reflects how I speak and If I do drag out a word in person I will in text. Now if someone does it for words that don't even make sense, then that's stupid.
Lol, I can only think of a few occasions I do that. It's a way of expression through text, a way to emphasize something. I wouldn't say "thisss" but I might be guilty of "sooo cool" or "Nooo!" :) I don't get too crazy with it.
I used to only do that only words like "hey" because I thought it looked interesting, lol. Now I have to type using proper grammar, otherwise it just bugs me. Although, I think people, probably girls especially, put extra letters to elongate the word and make it seem like they're enunciating it differently. Like when you hold onto the end of the word when speaking.