This is not the optimal solution, for exactly the reasons I outlined in my post - the fact of the matter is, the game is ultimately focused on multiplayer, not single player, and they don't want to put people in the situation where the character they've played with for a long time in single player, brought up in level, ect. becomes a liability if someone wants to play multiplayer. Let's say you find out a friend at school or at work has Diablo III in a month and you want to play together online. Sure, you can do that! IF you don't seperate out single and multiplayer characters.
You are not looking at the bigger picture. The above situation is a great discouragement to play multiplayer, and that is bad. Blizzard wants people playing multiplayer, and they want to make it as easy as possible for people to switch from single player to multiplayer, without compromising the integrity of multiplayer.
Being always online is really the only realistic way to ensure that this can occur.
You do not understand it; this is fine. But then you'll just have to trust those more knowledgeable when they say that this is, in fact, a good idea from the standpoint of both most players and the developers, and your loss is pretty negligible - the only real loss is if the servers go down or you have a very crappy internet connection. You know what?
That's too bad for you. Its a much better experience for the vast majority of players, and the change will make it much more likely that you will transition into playing Diablo III multiplayer (as, let's face it, you bought it already, and multiplayer is just a click away!) and thus play for longer and encourage others to play more, and likely encourage more people to buy into the game.
Its not some evil feature designed to make your experience crappier; it is designed to make your experience better. This is in contrast to most DRM, which is all downside, there -is- actual upside here for the end user experience. I don't even think of it as DRM, I think of it as a necessity to play the game.
Other games (like Assassin's Creed) where the single player and multiplayer experience are totally separate have no such excuses, but with Diablo III, it makes sense to do it this way.