An interesting post on the stable@ mailing list by Matt Dillon illustrates why are jail resource limits and other such measures important on todays machines. Actually, this also illustrates one of the "pro" arguments for virtualization.
After 10 years of it being conservatively tuned, I've recently increased the default read-ahead (vfs.read_max) in FreeBSD from 128 KiB to a whopping 512 KiB. And of course, I have received an e-mail from a concerned developer asking if that is perhaps too high :)
How much impact can read_max have I'll try to illustrate on this excellent example on a machine I'm currently configuring.