You can tell people where you came from. They usually find this interesting and there are also clients that have preference working with people close by or at least in the same time zone.
For web designers in particular, validation is imperative. If your aim is to build professional websites for customers then you have no excuse for not ensuring your website code is valid.
Apart from linking texts, images should be linked as well. The majority of users will click on images with the expectation that it’ll lead them to point someplace.
What if you don’t have any former customers to include in your portfolio? Simply create a theme from Word Press, design an icon set, grow a Twitter mashup, etc. There are lots of potentials and a huge difference between showing just one project as against showing nothing.