Here's a very good blog post comparing and contrasting DNN with SPS (SharePoint Services). It's long, but well worth the read.
Like anything, choose the right tool for the right job. Obviously DNN isn’t as scalable as SharePoint is and it can’t search file shares, web sites, Lotus Notes databases, etc. but then it also can’t be setup and running on an external web host where you don’t have console access in 10 minutes like DNN can.
Choose, but choose wisely.