well, so far I've only found one use for them. I never take both homeworlds at about the same time, I always take one first (the toughest) then move to the other. In the occasion the toughest homeworld has a core world with a Spire Archive, then I can use the extra knowledge while I'm working my way to the second homeworld. I often have to neuter some stuff, deal with waves (CPA's always enjoy spawning right in the middle of my endgame), etc, so there is quite some time between first and second homeworld kill.
But you're absolutely right, I'm not going to put a homeworld on alert on purpose, just for the extra knowledge. By the time I'm strong enough to deal with a core world, I already have pretty much all the knowledge I need.