Having an extra set of tubes lets you gauge the sonic deterioration of your main tubes. Such is good practice. Also, tubes do fairly often catastrophically fail (one channel goes silent... that kind of thing) --- and, by having an extra set, you can use those tubes to assess which of your tubes died (that is, if, for example you have 4 pre-amp tubes per channel and a channel died, you can readily swap out systematically the tubes to discern which tube died).
Tube Life
I’m using a phono pre-amp with a pair of telefunken 12AX7 and one 12AT7. I purchased these tubes several years ago and they were rated at about 50% life. Are tube noise and tube rush the only reasons to change out tubes? Everything is silent and yet I can’t help but think that several years of use should indicate some sort of wear. Any experience or suggestions will be appreciated.