Level vs edge triggering, usefulness of level triggering
As an addition to the other answers, another look from a practical perspective:

The level triggered interrupt is an indication that a device needs attention. As long as it needs attention, the line is asserted. A device may want the master to clock data out of the devices buffer. It may need immediate attention to prevent buffer overflow (so using interrupt is a good choice vs polling) but it wouldn't be practical if the device has to keep switching edges while the buffer still contains data. The master does its processing as fast as possible, clears the interrupt flag when ready and immediately recognizes that there is more to do.

Edge triggered interrupt is an event notification. When some particular thing happens, the device generates a pulse on the interrupt line and its done (fire and forget). The master just takes notice of it and goes on.

My point is, there is practical use for both.