I choose not to play D2's online section because it's just worse than SP in a functional manner of speaking. I get less where I get more in Singleplayer. I get less lag, I get harassed less, I don't have to pay attention to my messages. I don't have to care at all about bots or any such thing. I can mod and hack to my heart's content. I can even save-scum if I really want to.It's kind of funny that you would say that, safely nailed into your own single player shoes.
Now I play Counter-strike from time to time online. That's different. I'm not an "elitist" in any sense of the word; I don't refuse to buy games just because they're multiplayer and there are quite a few multiplayer games that I adore (BF 1942 having been one such great game). I've played a lot of online shooters: SW: Battlefront 2, CS:S, TF2, etc. but they just don't hold my attention as much as D2, X-Com, and so forth. I've played a lot of online/LAN RTS and TBS (Heroes and SC among my favorites). But I always come back to my bread and butter.
I understand why some people might find D2 multiplayer fun: they enjoy playing with their buddies and finding a greater variety of builds to come quest with you is good. Sometimes you even get to meet new people and build new friendships or rivalries. That's the distinction: I don't enjoy D2 multiplayer, but I recognize why its popular and why people enjoy it.