Here's what Chris Taylor, the lead designer of the original Fallout and the lead systems designer of Fallout Online said recently:
There are some MMOs that require grouping for mid-to-late play (FFXI comes to mind) and almost all MMOs force you to group for the majority of elder play.
Personally, and I'm not sure how this yet relates to V13, I like a combination of solo/group/raid content, with different goals and rewards for each. I have almost always soloed my characters to high level and then joined a guild for a raiding, grouping with friends occasionally along the way. Pick-up groups can be heaven or hell, depending on the players.
Certainly, we are planning on the proper tools to make grouping as easy as possible, while still allowing solo players to participate.
I've seen some discussion here and there about how many players is anti-Fallout (or, more generically, anti-post-apocalyptic). I'd disagree, of course, otherwise I wouldn't be involved in V13. There are plenty of precedents for grouping and social organizations. Even the Road Warrior had allies during his cinematic adventures. In Fallout, NPCs were always available to join your group and there were plenty of social organizations that had a number of members.
My vision includes the lone wanderer, erm, wandering through the wastes and a team of individuals working together for a common goal (like getting a truckload of fuel past some raiders). In any MMO, there will be times and places that have more people than others (social centers vs. instances, for example). It won't be all of one at all times.
http://interplay.com/forums/viewtopic.php?f=9&t=230&p=4258#p4258