In our estimation, the top 5 reasons to purchase portal server, include:
1. Search
2. Audiences
3. My Site (personal portals)
4. Flexible taxonomy
5. Shared Services
I know there are those out there that think Search is a lesser technology to navigation. I disagree, of course. Both are needed to implement a well-rounded portal. And Portal server offers some serious features that can't be found in a team site. These features are listed above.