 "tls/2" vs  "tls/1" fingerprints

hey @davidmcgrew, can you please help me understand the benefits and disadventages of using
"tls/2" mercury fingerprint over "tls/1" fingerprint, from the doc the only benifit i see that is mapping
extension id in  TLS_UNASSIGNED  to 0x003e    
and extension id in TLS_PRIVATE to 0xff00.
