Theres a few caveats i think...
- not all miners support stratum v2
- to support miners that dont, have to proxy
- allowing hashers to make their own block templates may introduce more overhead to verify?  

 Also the spec is still a bit in flux, so even some miners that support sv2 need to fall back to sv1 and then use a translator back to sv2.

But people should try it. 

 I mean miners will need to use a proxy anyway to incorporate their own work whether their miner supports Sv2 or not.