1) Assuming it's a mutual commitment. It means different things to different people.

2) Of all things, a D/s relationship does not imply equal status. (Please note, I'm not talking worth or value... just status between the two.)

3) It also implies, (to some,) that the status of the collared is always at risk. If I carry the "key" it means I can "unlock" the collar on a whim.