All of what you mentioned is available for phpBB.
Automatic backups - done.
Plugin upload - done.
Account switching - done (but yes, that last one will cost you the price of a beer)
Sitesplat would do well to make whatever they do available to the entire community, rather than supporting their own themes only, which does feel a bit like having their cake and eat it too - a bit greedy off the back of an open source project. Even if their stuff would be paid. Fair enough.
People are happy to spend on something they like, as long as that doesn't force them into using something that they don't like.