Get the right guy, and it works out good. I fill in for another bassist when he's overbooked, its his gig, I just help out here and there. That's maybe more of a problem, he's being "unfaithful" playing with other bands, where you just don't want to play too much.
Also, we had a guy filling in for our guitar player when he started getting some touring gigs. No problem. Eventually we just gave the fill in guitar player the gig, once he started "filling in" on 90% of the gigs...

Once the old guy moved out of the minors and into the majors, he was gone more often than not.
No hard feelings, the old guitar player keeps in touch, we get periodic updates on the road, I surf YouTube for him once in a while. Very happy for him, its great being in a major act, wish I was 20 years younger, I'd love to have done it back then. Not so much now when I have a real job and do music on the side.
But it does highlight the potential problems with that situation, if they like the replacement better, if you take too much time off, they may end up going for the replacement eventually.
The guy I fill in for, that band is a loose conglomeration of a large number of musicians, a band in the quantum mechanical sense... no definte members, but some play a higher % of gigs.. There's the primary lineup of VERY good guys (who of course will be playing often, miss many gigs), and a deep list of subs available.
Randy