A couple of points that I'd like to address.
Firstly, I think people are conflating singers attempting accents that they have only heard from other singers, with singers singing in an accent that was likely the one they were spoken to in their early formative years, until socialising with other children altered their speech. So,for me, Ewan MacColl singing with a Scots accent - having been raised by Scottish parents - is not the same thing as someone singing in an accent they have no real familial or cultural ties with.
You may not like MacColl's Scots accent, and I leave it to the Scots to judge its acceptability, but he didn't simply decide to adapt it from recordings.
Secondly, since Peggy Seeger and the Singers Club has been once again brought into the discussion, I think it's worth quoting from Peggy's
very well done memoir:
(After the London singer's performance of "Rock Island Line").."My gut reaction was to laugh. It was rude, outrageous - but I couldn't stop.I had to be taken out of the room. After the show, other resident singers insisted that we talk about it. Members of the audience were invited......one of the French audience members said he didn't like my singing of French songs. Stung, I passed the aggro on to Ewan: Well, I don't really like it when you sing 'Sam Bass' with an exaggerated American accent. Then Isla Cameron objected to Bert singing songs from Scotland. A trail of dominoes - we all lost, but in the process found a policy for our club and our club only. Repeat: The Policy was for our club only....(it) has been blown up out of all proportion. It was only meant for onstage singing....(s)ing whatever you want elsewhere..."

There is more in that section, and I realise this is Peggy Seeger's personal recollection of the event, but I do find it tedious that she and Ewan MacColl do seem to come in for more than their fair share of stick when it comes to this subject. (apologies if this section of Peggy Seeger's book has been quoted before elsewhere.)