But you can already upgrade your hard drives through the USB interface. Nowadays that's plenty fast enough even for external SSDs. You don't really need SATA any more.
I could definitely see why professionals or otherwise demanding users might prefer internal disks over external ones.
http://www.apple.com/imac/performance/
an alternative to the garden variety external SSD would be an external PCI-E card housing (e.g. sonnet or akitio) connected through a Thunderbolt cable. now that's a lot faster.
but, for that we buy an external housing in addition to the actual storage that goes into the housing. so this is more expensive than computers which let us install a PCI-E storage card directly inside the case. which Macs don't let us do easily or at all.