Arpan Shah has a lengthy posting answering a number of frequently asked questions regarding SharePoint Portal Server. Good arguments you can bring to customers when going into that type of discussions with them. To me, the top reason why I see people opting for Windows SharePoint Services/SharePoint Portal Server is 'integration'. WSS integrates deeply into the information worker products, SPS integrates well with the other server products. More work needs to be done on this since we can go much further with the integration (and we all know it will come), but compared to other similar products, this is where I think Microsoft is one step further than the others.