I see what your saying but the problems are ever present.
The fact you switch of plug and play does not require the manual setting of IRQ's in any respect.
The interrupt request lines will still be set but in a cascading order, i.e. on a first come first served basis - if you enable plug and play it will try to optimise this which is where the juggling comes in - as hardware comes and goes things move about and the drivers have to reset and reconfigure themselves.
When plug and play is off - when a device drops it does not affect the other devices as it does not try to optimise.
Best to leave it off and cascade, may be more inefficient in some instances but can save alot of headaches.