@marcusnetto: The great thing about EE (for me) is that I can fully tweak it to my design. During the last year I have learned a lot about how I can pull the most out of the EE possibilities. Going to a different Ecommerce solution and wanting the same flexibility usually means having to dig deep into the PHP (again) and learning the system behind it. With open source solutions this can be heavily frustrating and most of the time way to complex for just one project.
There are hosted solutions: you give the design to an eCommerce hoster and they pull it into the system, but those come at a different price tag and really tweaking it means lots of extra hours for the developers, driving up the budget.
So for me (and I think most others who put their time into learning to work with EE) it would be ideal if I could use it with my existing knowledge and also apply the same web-design and UI.
So the point lies more in budgets and dev time then the fact that you would have to log in twice (for me, that is)