If youre a high volume shooter, I think its definitely a sensible thing. Why wear out in practice what you are betting your life on? The more you shoot it, the closer and closer you get to a problem popping up. Will that be in practice, or a bad situation when you really need to count on it? Is it worth that worry?
Even if you dont shoot a lot, I still think its a sensible thing. You never know when something might go south, having an exact duplicate available just makes sense.
I shoot a lot and have two duplicates of what I carry. One for carry, one for practice, and one as a back up spare.
When you get down to what ammo costs are, the guns are the cheap part of the equation, and not really a big deal in the scheme of things, cost wise anyway.
If you're buying guns of known quality and durability, your probably going to be fine with just one, especially if youre not shooting it a whole lot. The one Glock I use in weekly practice didnt have a parts breakage until it was 5 or 6 years old, and in the 90K range. And even with the breakages, it was still working "sorta" (trigger return spring broke, gun still worked if you held the reset), not that you would continue to carry it that way and wouldn't want to get it fixed. It didnt have a failure that was more or less critical (broke a rail) until right around 150K. Glock replaced the frame and rebuilt the gun at that point too, and Im still shooting it today.
And just to put the ammo vs gun cost into perspective, What I spent in ammo, just based on my reloads costs in the 10 years or so I was shooting the gun before it broke the rail, I could have bought somewhere around 51 new 17's with the money spent on ammo. 35 or so if I used factory ammo. In the long run, the gun is cheap.