if you have a couple of bikes then it might be worth it coz your more likely to need it. But think about this, how many times since you've been riding have you wished you had a spare wheel? Not many id guess (tyres maybe but not an entirely new wheel). If you think about it you'd only need one really if you completely trash your other back wheel, and how often do you do that? I mean if one needs trueing your not going to swap them out are you.

It might be worth it if you kept one wheel set with road tires and one with XC, depends what you do i guess. Just ask your self do you really need another wheel, you dont want to buy a cheapy one really so you could end up spending a fair bit on a good wheel, and it might just be a waste of money if you hardly use it.

If you need a wheel while you wait for your hub to be fixed, see if your lbs will loan you one, or ask a friend maybe?