This one can be debated until you are blue in the face. There was a good debate about it last week on the IMBA FB site.
Logically leaves on the trail will protect from direct impact of rain hitting bare dirt.
But leaves are also organic, and therefore serve as kind of a sponge and can prevent trails from drying. A real thick bed of leaves can greatly affect this and be very slippery.
Leaves on the trail also obscure features and or hazards, be slippery on corners, and make it harder to get a "feel" for the trail. It's not bad, just different. There is nothing that says fall in Wisco to me more than riding in leaves. A totally different experience.
There are some trail systems that try and blow the leaves early. Then leave a thin layer of leaves to get a bit of the benefit, without the problem negative effects of too many leaves.
My take. And this is just me. There is no right answer. Every trail system is different. Type of trail use (fast, slow, etc). Amount of trail riders. And perceptions of the trail stewards all factor in.
I have not talked to Melissa, but with new trail work, protecting the soft soils of new trail work could also be a factor.
At CamRock, we have tried all of the above mentioned approaches. Deep leaves left a trail that never dried out, and was very slippery on the corners. When we tried light leaf litter, it just blew off from the affect of fast riders on most areas, and on the slower corners was still somewhat slippery.
With the really hard soils at CR, and the many fast sections of trail, we have opted to go with blowing the leaves. Getting the trail dry as soon as possible is the thought. There are times, mostly winters with little snow, that I could see the advantage of having leaf litter left behind. Basically protecting the bare dirt from damage of riders when it thaws. But then again, this coverage is never everywhere, so the trail will be damaged if riders are out on a thawing trail. Therefore we close the trail in thawing conditions throughout the winter if there is not sufficient snow cover.