A view as to the origin of the expression here http://www.phrases.org.uk/meanings/nail-your-colours-to-the-mast.html

I firmly believe that a singer ought to use his or her art to promote the causes he supports. It is that that makes those who do more than Bob Monkhouse or Rolf Harris "the all round entertainer".

I am not wholly clear about WIll's position but I note the OP says "Just in case anyone gets me wrong, I have no objection to musicians or singers holding strong political opinions and/or performing relevant songs or taking part in various projects. Nor, if they wish to make their opinions known at a gig etc. They have to make a considered decision on such matters and audiences will always respond accordingly depending on the circumstances."

Don's position however appears to reduce the "entertainer" to a cipher providing the opiate of the masses (another expression the view of which might have changed over the years).   To restrict oneself to such an anodyne role seems to me to reduce not enhance an artist.