I see this as a risk management issue.
Aluminum things crack at the welds. Its true for frames, and I suspect it is true for racks. There is some correlation with overall life, so don't expect a new one to crack anytime soon.
So the real issue is how does a crack affect you?
If you are on the Katy Trail, and your rack fails, its is probably no big deal. You hide your stuff in the bushes, ride on to the next town with a bike shop, purchase a replacement, and then ride back.
If you are on the Continental Divide route, and then nearest bike shop is three days off, then a cracked aluminum rack is a big deal.
You know your routes and usage. I think you can decide what risk is right for you.
I started with aluminum racks (Blackburn Expedition in the rear, JandD something in the front). I eventually migrated to Tubus racks.