Sorry to resurrect another oldie. Not trying to start (or revive) an argument, but I'd add my take, FWIW.

Firstly, I love the song in both rhyming and non-rhyming form. I have nothing against either, although I do have a preference.

I've always found the non-rhyming version, with its "Grows around the blooming heather" too repetitive to my ears: just a sprig too much blooming heather. I always preferred the "perfuming" version. I think it is valid. It certainly seems to make sense in light of various clues.

a) The rest of the verses approximately rhyme to the scheme that would allow "perfuming".

b) The "source" by Tannahill went:
An' the wild mountain thyme
A' the moorlands perfumin'

To me the question is not so much why should it have to rhyme as why should it not when the rhyme was in the "original" and works perfectly.

I have no objection to others singing it the other way, but I'll stick with "perfuming". It isn't in the least bit forced, it is natural and perfect and faithful to the source that inspired Mr McPeake.

If McPeake really did intend "Grows around the blooming heather", I find myself wondering why. The imagery is great, but for me personally it works slightly less well in the overall context of the song.

I have my asbestos drawers on.