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The problem with the original firmware was that its interrupt handler used a zero-page location to store the accumulator. AppleSoft BASIC uses the same zero-page location for its own use, which makes BASIC incompatible with interrupts.

Another problem was with in the original Apple IIe. Its firmware interrupt handler was not updated to handle the new auxiliary memory and 80-column video configurations.

ProDOS provided proper support for interrupts, even on older Apple IIs (needed for mouse-based GUI apps and realtime clocks), and the firmware was eventually updated in the IIc and Enhanced IIe.

Yeah I think nothing actually uses interrupts by default. But they're accessible through the expansion ports. Not sure what the problem would be though.

Serial interfaces could use interrupts, and it was necessary at higher baud rates.

Yes, I remember writing a print spooler using interrupts as part of my first job.