often i make them to fit the bed they are for- covering the mattress & box springs but not dragging on the floor- sometimes a pattern insists on ending before then- sometimes they are longer, or shorter...just depends on what i want at the time & the pattern/design i am making.
as for your (skirt) idea- if you want- go for it- as you said- it would be easy enough to trade out- change with any whim & you can use the fabric for other things if that's what you want to do-
make what works for you- that's one of the great things about quilting, you can design your own bedding to fit your desires & no one else will have what you have- it's personally yours.