Enter the computer room of the average mid-sized company and you're likely to see a few high-end servers running mission-critical applications, and next to them a bunch of desktop-style PC's providing secondary services. The reason is simple: cost. It's difficult to justify the expense of a proper server -- with redundant drives, fans, and power supplies, multiple network cards, and loads of memory -- for an application of limited importance and benefit to the company. Only mission-critical systems warrant that expense.
Further, many applications don't play well together. You may have a powerful, multi-processor server, but if you try to run several disparate applications on it, even applications that in themselves have a small footprint or light requirements, and you're likely to bring the server to a crawl, if it will run at all. So you run those applications on separate pieces of hardware, sized -- as closely as feasible -- to the application's need.
The problem is that low-end servers can cost just as much to maintain, if not more, than their high-end counterparts. Operating systems must still be patched, backups must still be performed, anti-virus and other security measures must still be maintained. And if a hard drive on a ...