There's got to be a FAQ entry on this somewhere.  If not, there
certainly should be.

In the Dark Ages of the PC, ~all devices on the bus used open drain
outputs to drive the IRQ lines.  Open drain outputs draw current.  If
you had more than one of these on a given IRQ line, it could cause
excessive current drain on the PIC and (perhaps) damage it.

Nowadays this isn't an issue, for a few reasons: different drivers on
the IRQ outputs, lower power consumption in general, and more tolerant
parts.  In fact, some devices (such as BusLogic MultiMaster SCSI cards
and many serial devices) are now specifically built to support IRQ
sharing, and sometimes even suggest it in their manuals.

So, making the kernel incapable of doing this would be quite wrong.