I'm not sure all would agree that when using the loupe/zoom tool in the Contact Sheet that the selection should change
Yes, that's true, but selections are very much at risk anyway when you start clicking around, aren't they? But to comfort the reckless users, a modifier key could be used while loupe-clicking, either to save the selection, or to select the clicked. I would prefer the first, the reckless the last
Indeed, in the current situation you can do this faster in Preview. But then, what's the point of having a Loupe tool in Contact Sheets anyway? To me, it feels as an incomplete feature. It is normal that an object is selected when you click on it, or added to a selection when you command*-click on it. This Loupe 'cursor mode' breaks that convention.
Making selections, sorting or cleaning up folders can only be done safely and effectively if you somehow mark the images accordingly. This is where PM shines, offering a variety of markers (tag, stars, colors) with simple keyboard shortcuts.
For almost every user this is the third stage of using PM (after ingest and the first round of IPTC), and also one where the speed of PM really is appreciated. I think that the Loupe cursor mode fits smoothly in this workflow if the clicked image becomes selected, or is added to the selection if the command* key is pressed while clicking.
I think that, in its current form, not many users actually
use the Loupe in contact sheets, but use the Preview way instead. Preview, though, is linear, while you can quickly jump through the contact sheet to check out the good-looking ones first. I think the Loupe tool would be much more used and appreciated if the image under the looking glass could be marked immediately, without any hassle.
*or its Windows equivalent, of course.