Intranet administrators know who their users are because the network is a closed loop. Software manufacturers use their sites to showcase their wares, so they expect more advanced end users. Artistic and personal sites usually have a specific audience in mind. Unless you own the Psychic Network, there is no way for you to know what browser version, operating system, connection speed, screen resolution, or color depth your potential customer has on his or her desktop. The goal of an e-commerce site is sales.
Sales occur when a customer buys something.
You should be aware that the threshold of boredom is dropping like a rock. If the customer can't see your navigation bars or won't wait 48 seconds for that fancy graphic to load, you risk losing that potential sale. I'm very aware of the strain Web site builders can fall under. The clients may want fancy navigation bars and wild animation on their pages. They might think they want crazy gradient backgrounds that change color as you scroll through the pages.