Rather than thinking of our interfaces as sculptures chiseled into stone, we should consider them like a script or musical score. They are going to be delivered differently every time they're accessed. We should embrace that rather than fight it. Well, first there is the user experience. No matter how well designed and usable your site may be, if nobody waits around for it to download, it will be wasted.
Speed is the most important factor in the success of any site.
Make a slow site, and people won't use it. It's as simple as that. But there is a purely financial aspect to it, as well. Bandwidth costs money, money that comes out of your bottom line as a company. I was once talking to a CTO of a very large website who explained that they had done a cost analysis of their bandwidth.