Back when Diablo II launched, I played it half way through and then got bored. 6 months later, once we had internet that wasn't pay per minute (ahh the "good" old days!) I made a new character on Battle.net and it was a revelation. Nothing beat the thrill of joining up with friends and going online to take down the Prime Evils! Obviously the characters from offline couldn't be taken online, because it was so easy to train a character. However to play on battle.net the servers had to be up, just like Diablo III.
In the end, the old Battle.net gave in to bugs, ending up with 1,000s of Stone of Jordan rings flooding the game, until they made a secret "Uber Diablo" that in order to spawn you had to sell 75 or so SoJ's to vendors. But for a time, there was actually an economy.
Enter Diablo III, and they have taken out the offline mode, which in my mind, had no worth. What's the point of loot if you can't show it off? This is one of the reasons why Torchlight felt like a waste of time to me as well.
Anyway, I know there are arguements about server down time, what happens when blizzard decides to shut down the servers, (which hasn't happened happened to Diablo II yet, in fact they recently had a ladder reset - Awesome website ahoy http://classic.battle.net/diablo2exp/) and that one asshole who apparently wants to play it on a plane every five minutes (screw sitting next to that clicking bastard on a long haul flight!), but honestly, if Blizzard patched in an Offline mode right now, would anyone here actually play it?