Tenon makes fine software products; I'm glad they are solving your problems. However, their software has to be run on some kind of hardware platform. If we spent our funds on software, we would still need to come up with hardware to run it on. So, this is not a feasible trade-off.
More generally, alternatives such as Post.Office and Mac OS X Server all share the same problem: they're not Mac OS X. We use vanilla Mac OS X on all of our other machines; running it on our server means that we only have one set of OS vagaries to learn and remember. We find this to be a compelling advantage.