When a machine starts using virtual memory, its performance drops substantially. If you've got a server whose throughput needs to stay high to keep up with demand, this is a disaster. But, worse yet, once things start swapping, it is really hard to recover.

By contrast if that machine had no virtual memory, the failures are much more obvious. Plus there is a good chance that the offending memory hog will die fast, and the server is likely to be able to continue doing everything else it is supposed to do.

Therefore if you have an expensive server, think about getting extra RAM and disabling swap. Or even just disabling swap.