I attribute the idea that performers must sing in certain styles[including an insistence on using both head and chest voice] to various workshops that certain uk revival performers[not Shirley Collins] have given, in my opinion these workshops were not based on uk traditional singing styles as sometimes have been claimed.
using a harder tone was an idea put forward by certain uk revival singers[not shirley collins].
I have spent many years listening to Traditional singers such as harry cox and fred jordan, bob lewis, they sang naturally, without the affectation of singing with a smile on their face or deliberately developing a hard tone.
how people choose to sing stylistically is up to them, but for anyone to claim it is in anyway connected with uk traditional singing styles is in my opinion not proven.
I think that there are a number of other revival singers who have been much more influential[mostly through song workshops] on revival style than shirley collins,one of those singers had a connection with the Critics group, and believed in the use of varying tone or using a harder tone. I repeat uk Traditional Singers in my experience did not do this.http://www.youtube.com/watch?v=YsxG06FMA-Y
or willie scott http://www.last.fm/music/Willie+Scott/The+Shepherd%27s+Song/The+Shepherd%27s+Song