Of course, the premise of the game is to form a group and go questing together -- One of the "M"s in MMO stands for "multiplayer," you know. But even hardcore players, and casual players more so, will want a breather from team play every now and then -- especially when the majority of players with which you can group are interested only in grinding through the same quest over and over again.
Should you play DDO 100% solo? No. There are other games for that (Oblivion, Neverwinter Nights, etc.). But to have the option to succesfully play the lone wolf from time to time? That's a facet of gameplay that DDO was sorely missing at launch. This is a good thing.