In a statement that was deleted in the meanwhile, two Blizzard employees talked about the reason to eliminate LAN mode from D3:
Karune:
"[...]
1) Pirated servers splinter the community instead of consolidating all players who love to play the game. Battle.net will bring players together in skirmishes, ladder play, custom games, and allow everyone the opportunity to share a common experience.
2) More people on Battle.net means more even more resources devoted to evolving this online platform to cater to further community building and new ways to enjoy the game online. World of Warcraft is a great example of a game that has evolved beyond anyone's imagination since their Day 1 and will continue to do so to better the player experience for as long as players support the title. The original StarCraft is an even better example of how 11 years later, players still love and play this title, and we will continue to support and evolve it with patches.
[...]"
Battle.net-User:
"All removing LAN support does is hurt legitimate buyers, pirates will have LAN and their free game.It might take them a while to reverse engineer it, but they WILL have it."
Bashiok:
"More so than overbearing/invasive anti-piracy measures that would affect everyone who buys the game regardless of how they're going to play it instead of just those that may want a LAN feature? I would doubt it.
I don't know a lot about it, Diablo III isn't really facing the brunt of the Battle.net 2.0 features just yet, but I think that removing LAN in an attempt to avoid more severe anti-piracy measures is pretty cool. We're saying "Hey, we're pretty sure you're going to love our game. The multiplayer is really the best part though. In order to get in on that that we'd just like to make sure you bought the game. Cool?"
(from http://diablo-3.gamersunity.de/text2104/diablo_3_im_battle.net_kostenlos_und_ohne_lan-modus_.html - German page).
I believe that this is just another move to promote Battle.net and offer premium services on it. I haven't played Diablo and Diablo 2 a lot but the few times I did it I enjoyed playing with my friends in the LAN mode.
Karune:
"[...]
1) Pirated servers splinter the community instead of consolidating all players who love to play the game. Battle.net will bring players together in skirmishes, ladder play, custom games, and allow everyone the opportunity to share a common experience.
2) More people on Battle.net means more even more resources devoted to evolving this online platform to cater to further community building and new ways to enjoy the game online. World of Warcraft is a great example of a game that has evolved beyond anyone's imagination since their Day 1 and will continue to do so to better the player experience for as long as players support the title. The original StarCraft is an even better example of how 11 years later, players still love and play this title, and we will continue to support and evolve it with patches.
[...]"
Battle.net-User:
"All removing LAN support does is hurt legitimate buyers, pirates will have LAN and their free game.It might take them a while to reverse engineer it, but they WILL have it."
Bashiok:
"More so than overbearing/invasive anti-piracy measures that would affect everyone who buys the game regardless of how they're going to play it instead of just those that may want a LAN feature? I would doubt it.
I don't know a lot about it, Diablo III isn't really facing the brunt of the Battle.net 2.0 features just yet, but I think that removing LAN in an attempt to avoid more severe anti-piracy measures is pretty cool. We're saying "Hey, we're pretty sure you're going to love our game. The multiplayer is really the best part though. In order to get in on that that we'd just like to make sure you bought the game. Cool?"
(from http://diablo-3.gamersunity.de/text2104/diablo_3_im_battle.net_kostenlos_und_ohne_lan-modus_.html - German page).
I believe that this is just another move to promote Battle.net and offer premium services on it. I haven't played Diablo and Diablo 2 a lot but the few times I did it I enjoyed playing with my friends in the LAN mode.