Yes, stop the weeds coming back and it will warm the soil slightly for planting/sowing in the spring. But even better would be to sow an over wintering green manure like field beans. These also hold back the weeds and stop the soil leeching its nutrients.

that's what i do, mainly because any seeds that you have brought to the surface will germinate. Things like dock and perennial nettles. Then when you remove membrane, the seeds will still germinate, but be small enough to weed out easily. I do it every year with my raised beds. but i mulch with chicken bedding and carpet.

I persuade the neighbour to dump his grass mowings at the end of the plot and then use this as a mulch after lifting crops. It gets dug in at the end of winter.

I think the opposite of the above but thats not to say they are wrong, its just that I have a different point of view.

I prefer to dig and manure my plots prior to Xmas, usually October / November then leave them open to the weather!

I see it this way; although I agree that digging exposes weed seeds I see these as winter feeding for the birds.

They can also find insect life, and possible pests, at the same time!

I also like the frost to break up the soil, as this often kill off some of the pests that may be in the soil, but normally not all.

It allows wind and rail to act on the soil and the rain water or thawing snow percolates into the soil and manure thus raising the moisture content of the soil and manure, meaning I rarely water (if ever) during the summer!

This pays off if there is a 'hose ban'

In spring just prior to planting I remove any pernicious weeds that have appeared.

Another advantage of having a few weeds appearing is; this tells me the soil is warming up, meaning I can plant out hardened off plants!

So you see I 'play nature' and it works for me!

So as I see it; covering the soil over prevents most of these natural events happening!

Have a look at this slide show I produced on how I prepare my soil each year;

http://www.thegardenersalmanac.co.uk/Data/Soil-Preparation/soil%20preparation%20ss.swf

Yes! Cover them! As you are in a wet climate there will be plenty of rain after you uncover the weed free beds in spring. Under the 'mulch' of black plastic all the weeds and mulch will break down and decay creating a lovely surface full of nutrients, and worms should busy themselves over winter taking a lot of the organic material underground.
Unless you are on heavy clay, I'm sure this is the best course to take.

I agree entirely with Teegee.
Early soil preparation and the winter weather to do it's job is the best way.