Like most people on the Net, I make some use of Yahoo! services. It's not easy to avoid it. Hundreds of thousands of people have a Yahoo! email address, just as many use Yahoo! Messenger, and Yahoo! Groups is "the new usenet" for many subjects. That's not even counting Yahoo!'s various other branding efforts.
Before I continue, I'm going to drop the stupid ! in the company name. Yes it should be there for accuracy, but good grief can we be a little less self-important? Thanks. Anyway...
The advantage of one company offering all of those services is that you have only a single sign-on to worry about. One name, one password, one bookmark, and you have all of your services at your fingertips. That's great... right up until you get butterfingers. It's also a single point of failure; one problem can bring down your entire PIM network.