I know which examples you're speaking of, plus, we often use supplemental dash tweeters in boats like Supras and Moombas where all the cockpit coaxials are behind you and lose their brilliance when underway. They're always attenuated to the degree they still contribute but cannot be localized. You won't necessarily notice them until they're gone but when absent they're sorely missed. It's just something that I'm particularly sensitive to and more than most. I would prefer to run hot tweeters down low over less aggressive tweeters up high because reflecting off the glass creates some stridency and, as mentioned before, I don't like the detached sound.
Running coaxials or separates with an additional set of dash-mount component tweeters really works out well if you bring the tweeters into balance and the collective tweeters are in balance with the midbass drivers.
I really like your idea of using the separate components only in the forward cockpit sweet spot where they're truly appreciated. Everything else is just ambiance and midbass fill to the driver. Separates usually have larger tweeters with a lower resonance and smoother response especially through the upper mids. Plus, without the tweeter supporting pole piece of typical coaxials, separates have a rigid dustcap, more surface area and produce stronger midbass output.
So those are my preferences. Taste and perception are subjective and we don't all agree. Since I'm hyper critical and lean towards the purist side I'm often in the minority and routinely yield to the likes and dislikes of the boat owner.