You make a good point. I have the spring pots.Did you get the fabric xl pots or the plastic? I got the discs with my 4pot plastic system and with the extra plastic pots I ordered. Would imagine with fabric pots just a disk at the bottom would allow roots to still go sideways and breach the fabric while a sock would go up the sides a bit preventing roots getting into the valve.
The plastic pot systems come with copper disks, and the spring pot (cloth bag) systems come with the copper pot socks.