You bring up good points. The big advantage I see is the ability to fallback to standard tools if you don't want to use SRV. If I want to debug something I can just type "dig SRV myservice.mydomain" and see the results. I can also just use standard DNS libraries to do just regular A lookups if you don't care about priorities. For anyone else that does, we've released a bunch of libraries so they don't have to do anything to start using it.

I also wouldn't say that SRV records are proprietary, they're an official record-type (that many DNS providers support).

Personally, I just don't see why anyone should be creating their own format when some other established and compatible one exists. DNS is already widely used for domain/website discovery, why should that not be extended to inter-datacenter discovery? That's obviously rhetorical for this conversation, but that's what we asked ourselves.