They still follow the same philosophy, yes. Blizzad have always cut out features when they feel that they can not make them good enough for the release. Heck, "WarCraft 3" was supposed to be a more RPG styled game with much more focus on individual Heroes than the RTS part of the franchise, at that time (at least for singleplayer, as far as I know). Though, they toned the RPG down and made it work a lot more like the previous games worked.
Another example would be the chat rooms in "StarCraft 2". The game played like a charm on release and you did not really need the chats to enjoy the entirety of the game. Delaying it just for one single feature not being done would have been just annoying and would just make us wait longer than we actually needed.
This is also the case for the "Diablo 3" Arenas. They feel that the game is really solid as it is and they will add the feature some time after the release once they got all the issues with the system resolved. Trying to fix it all for the release would just delay the game even more and people are already at the top of their seats. They are aiming for a release now and the features not quite ready yet will be postponed.
Also, Arenas are an end game feature. People will be quite busy with other stuff before they will worry about what to do at end game, so Arenas will not be needed yet. If they release Arenas a couple of months after the actual release, it will have a minor impact on how people will enjoy the game, so it is an acceptable loss, for now.
As long as it will make Blizzard release "Diablo 3" faster, I will just be happy. =)