There really isn't any one webpage that I want to go to first thing every single time I run IE (or FF or Chrome). This is why I have Google set as my homepage. I open my browser and there's Google, waiting with practically no load time. If I want a web portal, I click on the iGoogle link at the top-right; if I want to go to a website I'll find my own way there.
I don't need a portal most of the time, and having my browser open up to a web portal filled with banner ads, Flash animations, Java and whatever-else-have-you every time I run it would be unacceptable. I use Google as my homepage because it provides more functionality than having IE/FF/Chrome open to a blank tab, while at the same time having basically no load time. As far as utility is concerned, not much tops Google as a homepage.
Still, Google has iGoogle, a completely customizable web portal interface that anyone can use. Yahoo's advertising is playing up an experience that they don't have a monopoly on providing. There are other strictly web-portal sites that provide functionality on par with Yahoo's web portal, and this commercial doesn't give any argument for Yahoo's web portal being better than anyone else's. The commercial does seem to say "we're better than Google (even though they don't specifically name Google) because we're a search engine *and* a web portal," but it completely ignores two facts: 1., that Google provides a less cluttered and more efficient search experience, and 2., that Google has its own web portal, iGoogle.
Essentially, the commercial is a poor showing, because all it is going to accomplish is getting people who already use Yahoo to keep using Yahoo for a while longer.