Interesting topic. Here's my thoughts. No science behind this other than common sense. Imagine your bait is a bath spnge, when it's fresh out of the pan. Full of water, but also freshest. That's a fresh boilie for me.
Now imagine you leave it a few hours/days whatever until it's dry. If you rehydrate with water you're back where you started, almost. Of course with a boilie the ingredients are not quite as fresh, but with MPG or similar, close enough.
Here's where it gets interesting though. What if you don't use water, but instead another liquid? You can now introduce no end of variety as you re-hydrate. Even better, if done hours or days before fishing it's going to look washed out, but is pumping out more attraction than the fresh boilie.
So I think the last option best of all given time and opportunity to do it. Air dryed, then rehydrated with added attraction. If you do that several times and introduce powders when the liquid is still moist you're boosting your bait even more, to my mind anywy. There are a number of anglers and baitr companies doing this now. Good use of time and energy or gimmick? Time and captures will tell I guess.