Document management does not scale beyond a single server, but scales great within a single server. For example, a quad Xeon machine with 4GB of RAM works great for a document management server that has about 900,000 - 1,000,000 document, but if you need to store 50,000,000 document and want to have them all in one single workspace then it does not scale at all. If you need a scenario like this, you need to plan your deployment right and it should scale for you, it just does not right out of the box.
If you are using your server as a portal and search server most for the most part it scales great. You can have many different servers crawl content sources and have separate servers searching and serving the content.
If you have < 750,000 documents per server and fewer than 4 content sources and fewer than 50,000 users, SPS should scale just fine for your needs with the proper planning.
What are the actual advantages of SharePoint Portal Services (SPS) over SharePoint Team Services (STS)?
SharePoint Portal Services (SPS) has MUCH better document management. It has check-in, check-out, versioning, approval, publishing, subscriptions, categories, etc. STS does not have these features, or they are very scaled back. SharePoint team Services (SPS) has a better search engine, and can crawl multiple content sources. STS cannot. STS is easier to manage and much better for a team environment where there is not much Document Management going on. SPS is better for an organization, or where Document Management is crucial.