""Flag-shagging" is the SNP's only raison d'etre."
If you you mean they're nationalists, then sure.
But as someone who studies political iconography fairly seriously, I'm often struck that they don't actually use the saltire that much. it is absent from their historical campaign material and publications, and even more recently, their party emblem and colours are more common than blue/white/saltire.
If I had time to sample carefully, we might well see different patterns at different elections, but my sense from a pilot study is the SNP has not overplayed the saltire.
Where it seems to function more is, as you note, as a unifying image across different branches of the independence movement, perhaps because it is not closely identified with any one party.
I suspect a case could be made that most uses of the saltire in our daily lives are actually more civic than political?
anecdata: my primary school in canada's twitter account still shows a picture of the school with a saltire on the flagpole.