I know that D3 has a campaign, but it's integrated into the online mode (on Battle.NET), that means there is no actual offline singleplayer.
You need to login in with your Battle.NET account before you can access your characters and play the game.
Seems to me like pirating D3 has 0 effect, you can't do anything with the game without signing in, which (I'm guessing) isn't going to work without some kind of legit key or what ever else.
I can see some pros (for Blizzard anyway) in this online only idea;
No (or atleast very little) piracy, much less cheaters, etc.
But for players there are some serious cons;
-no internet acces = can't play.
-no local characters: can't make backups or do quick-fun "god chars" for singleplayer use.
-no mods.
-if you're playing hardcore, lag deaths cannot be avoided.
-possible connection issues (the open beta weekend had a lot of them)
and honestly there are a bunch of other points.
All in all, I'd say the online only is a massive let down.
Djiel's comments