They is not to make soloing worthless, it is to offer a mix of content. What WoW and Rift both did was to offer a leveling experience that allowed you to solo all the way to max, but at the same time offered you a big reward (fun, items, diversity in experience) in playing through instanced dungeons. If you only allow people to effectively play grouped, you aren't going to be attracting a sizable audience for a sustained period of time.
There are too many other gaming/entertainment experiences out there competing for people's attention for them to want to have to spend a material portion of their gaming time doing little to nothing while getting a group together. One might even say that by offering compelling solo content, you keep people in the game, doing something, remaining available for invites to group content. Otherwise if a person logs on and finds his friends are already in a dungeon run or are offline, he is apt to log off or switch games. Having the mix of content is a win win: the player gets to play an enjoyable game while being available for potential group invites.
Solo content also helps your newer players by allowing them to progress where otherwise there as nobody to group with, obtain gear needed for advanced grouping content, etc, without having to have players much further along stop their progression to come help.
Think about the ecosystem of players: What are all of the things a player needs to do to have fun, remove as many obstacles as possible, and let them go hog wild. There is a reason we don't see games that require 20-30 minutes of regging up time any more: it was boring as hell and made sessions dominated by administrative work and not playing the game.