Not that useful until there's no common "Import Bitcoin Core wallet" feature in Electrum, Armory, Android Wallet etc. Blame me if the feature exists and I couldn't notice.
Well, Bitcoin Core has a very simple format. You don't need any developer support to understand it:
label=%s # addr=%s (in-use keys)
reserve=1 # addr=%s (for reserve keys)
change=1 # addr=%s (for change addresses)
Time format is ISO8601 (%Y-%m-%dT%H:%M:%SZ), key format is WIF (importprivkey format).
At a certain point the Android Bitcoin Wallet export format was the same except that it was encrypted with AES.
I actually agree with you though, it would be great if different wallets had a standard export format. The first step would be writing a BIP. If the proposal is sensible I don't think much is needed to get wallet authors to implement it, as the need for this is kind of a no-brainer.
For the different deterministic wallets this is more tricky as they (currently) use different ways to generate the keys. If they all used BIP32 it'd be a lot easier.