Ftrace has found its home currently in debugfs. And the main reason is that we expect to be able to change the output when we feel fit. Ftrace is still new, and we are still learning what the best ABI should be. Until we have a stable ABI, I do not want user space tools to rely on it yet.
What would be nice is to have another filesystem like debugfs. Maybe a "stablefs" that things can move from debugfs to stablefs when the ABI has been set in stone. That would be a nice way to let user application developers know what they can rely on.