Actually, Halifax Tar I'm in agreement with you.
I was a purple trade (Adm/RMS Clk) and spent the entirety of my career, with one exception, in Army field units.
The one exception was an air force squadron I was posted to as a freshly promoted Sergeant, right out of the Airborne. To say I was a fish out of water is an understatement. I just didn't "fit" - nothing wrong with the Squadron, it was just that I was an "Army dude".
During my career I saw many air force and navy folks posted into field units. This usually worked out fine at the Jr NCO level, but could be the kiss of death for a Sr NCO. Many seem to forget that each element has different expectations regarding what secondary duties their people are expected to fulfill. In the case of an army RMS Sr NCO, you are expected to run a CP, site and man defences, along with all the other "field" stuff that comes second nature to one raised in that system. I imagine there are similar duties expected of Petty Officers and Chiefs on board ship - nothing to do with the trade, but everything to do with the element. Some folks I served with adapted adequately to their changed environment, but many others did not - which caused them to be miserable, their troops to be confused, and adversely affected the operational effectiveness of the unit.
I would hasten to point out that the above applies mainly to "pointy end" units - ships, army field units, and flying squadrons. I don't think (although I don't have any personal experience with it) that there would be much difference being a purple trade at Garrison Edmonton, or CFB Halifax, or CFB Cold Lake. There's much more to being a sailor than meets the eye, no matter your primary trade, same for soldiers, same for airmen. The "purple" idea works at static locations, but it sets people up for failure when at the "sharp end", no matter the element involved.
Roy