Well I think we need to define "pro" here to debate that further. Notwithstanding the folks in this thread, Apple's point, which they've been making since the original Mac 128k, is that 99.9% users DON'T need that expansion. How many people in the community at large ever even open up their mini towers and put in RAM, let alone graphics cards or new drives? If you go into the houses and offices everywhere, mostly you find mini towers with 4 or more USB leads falling out the front or back, connected to cameras, keyboards, mice, iPods, speakers etc etc. When are folks going to realise that the days of the "average" PC user, tinkering around with PCI cards, Soundblasters, "3d accelerators" etc are basically over. PC Gaming is already a niche with every man and his dog now playing consoles. The vast majority of users are quite happy with not having the ability to fiddle with the internals. As as far as "power users" such as gamers go, they usually throw the whole lot out to upgrade anyway. People agonising over CPU sockets like they are really gonna upgrade. Get real, you'll throw out your MB to get USB3 or something else soon enough. They will make sure of it.
The iMac is an appliance. As eMeek77 says, non-pro users still need their "appliance" to be fast especially as we are increasingly playing with large digital images and movies, hence the i5/i7 iMac. Macs keep their value a lot better than windows boxes, so what's not to love.
