First off, I am a track guy.
Pre-production models of the 1st Boxsters did not have a spoiler. Porsche discovered at high speeds (WELL into triple digits) the back end began to lift.
So as a last minute addition, the rear spoiler was drafted into production. It was defn needed on a vehicle that supposedly has a 160-170mph top speed. I would assume Porsche chose 75mph for it to deploy because of research they did, but don't know that for a fact. It may deploy at that speed because it would have a smaller, less adverse effect at that speed than popping it up at 100, IDK. The rush to get the problem solved before production began may be why the spoiler looks like an afterthought.
When I'm tracking my CSS, I manually set my spoiler to up. This is because the tracks I drive incl runs up to 130mph and several corners in the 30-50mph. The spoiler would be going up and down multiple times per lap. Since it's a mechanical device, I don't want it failing on one of those 130mph blasts.
Would not having the spoiler come up at 75 or 85 cause you to crash violently? Probably not. Would it provide any effect by having it up when driving 35? No. Can you drive it that slow with it up? Sure.
Burg - on my 987.1 CSS, when the spoiler is manually deployed, the spoiler stays up until you put it down manually. It does not operate on speed. Can't imagine they did it differently on a 986.