I have used fleece and nets but not together. I found that both were fine until it snowed, when the snow was held by the nets or fleece and so crushed the plants beneath - this was 2' of snow which without the nets/fleece to lie on would mostly have fallen to the ground. For shorter plants I make hoops of blue water pipe to hold the material up, but for taller plants I use canes with tennis balls on top (the most likely to collapse under the weight of snow). Both are held down by long canes either threaded through the net or placed on top of the fleece, then pinned down with wire pins made of old fence wire.
I think the problem is that in Britain we tend to have very wet snow - great for snowballs - which doesn't drift down through the mesh as it does with dry powder snow.
Other than in that situation, you don't need nets to hold down the fleece, just strings next to each hoop passed over the top of the little tunnel formed, and attached to tent pegs.
So, no - this year I won't be using mesh or fleece outside but will leave my brassicas to cope as best they can (and probably be eaten by rabbits) and will grow other crops inside my polytunnel