I know that long term Windows Phone users will be aware that the People Hub does have some interesting omissions in terms of features and clients. It's not enough to stop me using the hub, and in regular use I don't have an issue with missing Twitter DM's or not having access to geo location networks such as FourSquare. But when you go into an environment such as SXSW, where Social Media is king, these missing elements are very quickly highlighted.
The advantage of the Windows Phone People Hub is only as good as the content it can show and interact with. At SXSW I've found that it's lacking, and while the ability to do the same tasks with third party applications means functionally the hole is filled, it's not an elegant solution, and it diminishes one of the main differentiators of Windows Phone.