Source, please? Have you spoken to the designers?
There is no design benefit of ERVs, and the mechanism for their creation by retroviruses is known.
Your argument could be used in any situation to cast doubt on a claim of lack of design feature. For example if I claimed that the specific pattern of craters on the moon was not a design feature you could say "source, please? Have you spoken to the designers?". But would the obvious lack of a soruce, or lack of transcripts with the designers somehow lend any credance to the idea that the pattern of craters on the moon are design features? No of course not. The claim for a lack of design feature is based on the lack of any known design benefit coupled with the known natural creation of those features. The burden is on those who claim something is a design feature to show how it can be such a thing. I don't see how ERVs, or moon craters provide any design benefit.
So what? Literally, so what? Unless you know in detail what the designer had in mind, you cannot say.
My point was the logical inconsistency of allowing for the nonce, for the sake of argument, the existence of a designer, and then jumping to conclusions about what a designer's purposes MUST be.
I agree that from an engineering perspective, ERV's serve no apparent purpose. But for all we can verify of designers (nothing), one of the designer's teenage kids and their friends might have sneaked into the development lab one night and put in ERV's as a prank.
Cheers!