Enabling it should display a big scary warning about handing over partial control of your machine to greedy faceless megacorps who will throw you under the bus in a heartbeat if that makes them money. Maybe add an illustration of a robot terminator, to really drive the point home.
Yes, I agree that a default Debian configuration should be clean. But I also know the reality of needing at least Atheros (Wifi), Intel (Ethernet), and NVIDIA firmware blobs to get my tower to boot correctly. Not including any firmware repository on the CD or on a fully installed system is just a recipe for users to get frustrated while they are trying to follow a half-outdated tutorial on one of those spammy tech support blogs.
So in my opinion, the most common blobs should be cached by default, but not activated by default. If the machine boots without them, they can be deleted upon first reboot. If not, we can let the user decide if they are willing to compromise their principles in order to boot the system ;)