If we did have a CASS-like system in the EU it would be beneficial but don't for one moment think it would be free.
Speaking from a UK perspective, who would pay the taxes? Commuting crew are not exempt from this and along with airport charges this mounts up to quite a lot. Any airline that cares about their bottom line would not pay these taxes and charges just so you could have a free ride. If you look at a LHR-Europe trip one-way, APD is approx £20 and HAL charge about £30 just to use the airport. You'd need local governments and airport authorities to exempt air crew travelling off duty from these charges. Certainly in the UK there's not much appetite for that (I wish there was!)