Sure, there are some situations where you cannot connect to internet, but it's really in minority.
The problem with "always online" DRM is not that "there are situations that you can't connect to the internet". The problem is that you rely heavily on the other site and the medium for it to work flawlessly.
Let me give you an example (there is a problem with it and I reckon right from the start): It's a matter of only 2 or 3 years that Blizzard changed the maintenance cycle of their oceanic servers in WoW to coincide with the Oceanic timezone. Before that, any WoW player in Australia (for example) would get home on Tuesday and find that the maintenance cycle had just started. And that it was later extended another another for whatever reason.
Reports of Blizzard losing connecting to whole networks (like AT&T) happened even recently.
Now imagine that you finally got a day off. It's a rainy day, there is nothing going on, so you decide to play Diablo 3. And then you get the news that the servers are down or that there is a problem with the connection of Blizzard and your provider or Anonymous got ripped off in some Real Money AH and decided to bomb Battle.net login servers. Now what?
There are too many variables to give you an 100% fun experience with it.
I have no problems if they required a Battle.net account with a registered Diablo 3 key in it to play multiplayer (i.e., the lack of LAN play). My problem is that I may want to have a quick fix and I have to go to a check list to make sure I'll have fun.