Seamless Integration for Web 2.0

I keep hearing people talking about seamless integration. They keep saying that they don’t mind using a third party service to add something to their site but only if it can be integrated seamlessly within their site. In other words, whatever service they use, you shouldn’t be able to tell it’s a third party service. Now ask yourself, how many so called Web 2.0 services today can tout this? Why is this happening? Is it really a limitation of the technology or more a case of these services placing a higher importance on their brand over their customer’s needs?