Using a completely new string format in the same field (or removing it entirely) breaks a lot of sites that'll never be fixed.
Freezing it prevents this. And if we're freezing and creating a new system then why not go for something queryable without all the baggage?
I expect no UA header to be even less usable now that sites are more paranoid and app-ified, so instead I use a random one. That still confuses some sites...