4. If you put ads from the start, you get to design the Web site with the ads in mind rather than adding them later in whatever space that happened to be left empty.
That's an excellent point. My follow-up question is: how easy/hard is it to find advertisers for brand new sites? I know that AdSense wants you to have 100K uniques (or something like that)...
Adsense and adbrite have no minimum traffic requirements and cost nothing to get started in. You just won't get a payment until you've accumulated $100, which may take a while.
