1) People who are in the service overseas that don't have an internet connection or one that isn't really viable.
2) People in rural areas where high speed internet is simply not available or places like hotels and during flights
As for piracy, you're never going to stop it. Build a big mousetrap and someone will build a faster mouse.
And finally, lag is still an issue for many players. People who die in HC do so because of internet connection issues, not because of lack of skill. That and rubberbanding. Yes we are living in the 21st century, but that doesn't mean we have Star Trek quality data transmitting over worlds in an instant. My office building in NYC still have 19th century copper wire underneath the infrastructure and so as a result our internet inside our cubicles is super slow. In 1999 cable/DSL internet was just starting to hit the mainstream market, but people still were using dial-up. Can you imagine if D2 was online only??? Anyone remember Fantasy Star Online for the Sega Dreamcast? My friend was one of the first to buy this MMORPG for a console that had a 56K modem in the back. Yet when he played the game the lag was unbearable.
The first night I downloaded D3 I couldn't play because the servers were fried. If this were an MMO I would have least understood. After all, Blizzard had the same problem in Nov of 2004 when WoW went live.
But this game wasn't designed to be a MMO, and yet it almost feels like it without the trinity system and raids.