Let's face it, people don't like to read small print. That's why they have to be told to read it before signing a document. That's why it's in small print in the first place.
So why do so many web sites use small font sizes for their main text?
If the viewer doesn't like small type (or can't read it), you've just lost a reader.
How about viewers who do like small type? Presumably, their browsers are adjusted accordingly, and the type on your page is now too small for comfortable reading. You've just lost more readers.
What about that growing segment of the population, the elderly (and even not-so-elderly) whose vision is not what it was?
What about those with old, small, poor quality, badly adjusted, or dying monitors?
So why do they do it?
Because the designer likes it? Your site should be designed for its viewers, not its designer. Get a new designer!
In order to encourage viewers to use a text browser? Hardly a good idea after investing a lot of time or money in graphics for the site.
A browser's default size is intended for normal reading, and users set their browsers so that their defaults are comfortable. Make them uncomfortable, and you lose them.
If I really want to read a site with small type, I can, by changing the browser's font size or my screen resolution (an annoyance; few sites are worth the effort) or by using Lynx (a very useful, fast text browser; you lose the graphics, but that can be a bonus. Unfortunately, some sites do not display well in Lynx).