My understanding is that disks often fail when a head touches the surface, or a piece of dirt gets between the head and the surface. Once that happens, more dirt is produced, increasing the probability of more head crashes, leading to a failure cascade. As a consequence, once one of my drives starts to show unrecoverable errors, corresponding to damaged surface areas, I replace it while it can still be read.
The spare platter strategy does nothing to reduce this failure mode. In fact, all modern disks already have spare space for bad block relocation.