From my understanding Ublock Origin in Tails will add to the fingerprinting(websites can tell) and it opens up a new attack vector. The filters if compromised could add javascript to any website. Also, someone could think they are improving security and add an HTTP filters on that list which would open them to an attack whether it was comprimised or not. It obviously comes out of date, if someone updates the filters that will add to the fingerprinting(quite possibly you could fingerprint ad blockers by how up to date they are. Did the Tails developers lose their minds?
Tor users also generate revenue for websites by showing ads. If you block ads you add a huge incentive to block Tor users that wasn't there before.