UPDATE: some say that's because XMPP was too encompassing of a standard (if a format allows to do too much it loses usefulness, like saying that binary files format can store anything). IMO that's not the reason, they could just support they own subset. They scrapped interoperability for competition only IMO.
I don't think that's IETF policy. Individual IETF working groups decide whether to request publication of an RFC, and the availability of open source implementations is a strong argument in favour of publication, but not a hard requirement.
If the IETF standards are sometimes useful, it's more a matter of culture than of policy.
However, I think DER is good (and is better than BER, PER, etc in my opinion). (I did make up a variant with a few additional types, though.)
OID is also a good idea, although I had thought they should add another arc for being based on various kind of other identifiers (telephone numbers, domain names, etc) together with a date for which that identifier is valid (to avoid issues with reassigned identifiers) as well as possibility of automatic delegation for some types (so that e.g. if you register an account on another system then you can get a free OID from it too; there is a bit of difficulty in some cases but it might be possible). (I have written a file about how to do this, although I did not publish it yet.)