You could just build your own application server from the Java APIs and do your own IO and networking.
In fact, why use the Java APIs? Why not write your own programming language, create compilers for every platform in existence, and then write your own application server using your own language. Then, you could build your own portal sever, using your own application server, written using your own APIs, from the new and awesome programming laguage you just invented.
Yeah, you're right. In fact, my website, www.scja.com, has a bit of a portal look to it. Alot of people ask me what portal server I'm using, but I don't, I just use tomcat and arrange content in portlet like views.
It's like Struts or JSF. Why use them? You can do all that stuff on your own. But, why do something on your own, when someone else has done it, and probably done it better than you or I could?
Portal is just a technology that simplifies the aggregation of vast amounts of disperate data into a single, easy to manage website. It also facilitates personalization and customization of the user experience.
If you can do that on your own, without a portal, then there's no need for a portal. If the portal can cut down your costs, and speed your application to market, then it's the right choice.