I know plenty of people rationalise it on depreciation.
If you'd sold your 580 just before 680 release you would have got about £250-300 for it second hand. Buy a 680 for £400. Net cost £150. Let's say you do that for two years (not a ridiculous GPU cycle), then your net cost is £300.
Alternative: Buy a GPU, two years later, sell it for maybe £100, and buy the top of the range GPU for £400... Net cost £300.
Works out the same, but you end up top of the range for longer with habitual purchases. To be honest though, if I had that much money, I wouldn't run more than SLI. For a start, after SLI, scaling becomes ridiculous, and I doubt I'd ever be using more than 3 screens, which to be honest, for most games, a single card can run most of the games I play happily....
kd