If you need it done in a hurry, you don't use a mainframe. Their performance is actually quite poor compared to pretty much anything else, as is the cost per transaction. In general, if you need a machine where the hardware never fails, and you can't get the level you need with a cluster, you use a mainframe. Of course, it's still human error that bites you in the ass every time anyway. These days, you really only use a mainframe if that's all you have.