Well, in this case another attacks vector is possible - creating fake evidences. Unfortunately, authors of all proposals like that dont' describe details, so it's hard to propose concrete attack.
It is impossible to create fake evidence - you will need the private key of the pool's owner. 2 digital signatures of the same pool for 2 different blocks at the same block height are the evidence. The same block height must be digitally signed twice by the pool. No one can do this other than the pool's owner.
Private key is not needed sometimes. Evidence is just some bytes + signature. So attacker is going to find any signed byte sequence of needed length in the history to re-publish it as "evidence". Ok, bytes are height+hash(as in Tendermint paper), so height could be checked, so not every message is appropriate. But you can't verify hash.
But okay, that's not a pools-specific issue. Problems I see with pools:
1. Blockchain bloat with mining rules inclusion
2. Even harder to make SPV client(no SPV is possible for PoS of today , but there're some proposals e.g.
https://github.com/billlwhite/ledgertheory ).
3. Ok, we'll get 2-3 pools signing all the blocks. It would be centralized as hell system. What's the decentralization incentive?