When the forums here with SMF for FREE were upgraded to 1.1 the Administrator stated that we would be using one theme from here on out. Installing other themes when having several servers (and there ARE many you guys are not aware of) is not some easy task. There is more to it then that.
yes maybe but possibly smf could make a theme converter for these themes?

EDIT: like smf could make a website where you upload the zip file for the theme and it gives you the headers and footers and the css code

There is a lot more to most all of the themes then the css file itself. There are files we do not have access to on the system that need to be edited with some of them. Using different themes means smf would have to ensure that everything works with every theme on every server (as mentioned above) Themes are not just a css file, headers and footers (in fact headers/footers are only used when we are trying to do remakes of them becasue it is the only way WE have to do it)
I honestly do NOT see us having the ability to use different themes. We can change stylesheets and as people learn more and more about how to write new ones/edit the css I would expect we'll see even cooler stylesheets for us all to use.