I see testnet as a tool for a certain type of testing. Its existence is likely a consequence of Satoshi not writing unit tests and having automated integration tests, but creating a shadow chain to try things out, mostly manually.

I do not say testnet (as we know) would not be useful for certain tests. E.g. as we developed myTREZOR with slush it was useful to have a shared chain with worthless tokens and transactions we can both refer to. However for our automated tests chains-in-a-box are better as we can easily create and exactly re-create wierd situations on-the-fly.

While talking about BIP32 hierarchy use, several people argued to use a level of the hierarchy to identify the chain the key is used on. That level could identify testnet but as well an alt coin chain.

Above leads me thinking that testnet is far less important than to be addressed in every future BIP.