If you ride your good bike your going to wear it out. It makes more sense to use a cheaper bike, which is cheaper to maintain with cheaper replacement parts.
For example, Say you do 2000 miles a year with your other half and you go through a set of rings, a couple of chains and a cassette. If you were using your good bike with full xt your looking say £200, however if you were using a cheaper bike with deore throughout it would cost you say £100, that doesnt include increased wear on suspension forks and bushings and pivots.
why trash a good bike when you can ride a cheaper one.