In her call for designers to be more fluid, Ros Horner lists 10 reasons why designers should build their own sites. I think #2 might be the most important:
You’ll understand how it feels: How it feels to be so proud that something actually works that you momentarily don’t notice if the kerning is off or the padding is inconsistent. How it feels to see your site appear when you’ve redirected a URL. How it feels when you just can’t work out where that one extra space is in a line of PHP that has killed your whole site.