Single source hardware is an asset to server reliability, not a detriment. Compare linux uptimes to AIX, Solaris, or HPUX, and there's no contest. Having 1 company on the hook for everything helps make things run way more smoothly. The trend towards linux in the data center is a detriment that mirrors wall street, with short term costs overruling long term benefit.
As they used to say, "nobody has ever been fired for buying IBM." I think its safer to buy from a company that has a name and decent reputation in the server business is safer than Apple who has a recent history of pulling out of the business and never offered the variety of hardware and services of any of the major players. Even ignoring the high end players, I'd say that, for instance, a Dell server with Red Hat Linux would be a safer investment than who knows what from Apple these days. Ignoring the small choices available for Apple servers today, what happens if they discontinue the Mac Pro altogether? What if Lion Server is "defeatured" to make it easier for home server setups?