All season tyres?

Goudy

All seasons are a compromise.
(Then again, winters are in summer and summers are in winter in some areas).

All seasons use a different mix of rubber compound.
There's more silica in it to help keep it flexible in really cold weather just like full winters.
Below 6 or 7c is where this makes a difference.

The thread pattern is a "cross" between summer tread and full winter thread.
They tend to have wider, deeper grooves and a number of sipes (small rippled cuts in the blocks that helps grip snow and ice).
Full winters will have a much greater amount of sipes.

The thread pattern is often V shaped, so the groove are usually diagonal.

The compromise is the different compound of rubber can wear a little more quickly.
Wearing down the sipes in summer will obviously effect it's ability to grip the snow in winter.

Due to the pattern and the deeper, wider grooves there's also slightly less tyre in contact with the road in normal conditions, which is why you get reports of increased braking distances on most all season tyres. 
In the snow, this is the opposite as the deeper grooves and sipes create a larger contact area with the snow.

And of course the deeper, wider thread can cause the thread to move about a bit more between the road and the tyre carcass.

Are they worth it?
This is probably as close to an answer as you'll get.

400ixl said:

Where in the country do you live?

Do you live in an area that tends to spend a lot of time below 6 or 7c?
Suffers from enough snow and a certain amount of untreated roads in winter?
Have to make those journeys on the days you get those conditions?

If yes they maybe be worth it as they should workout a cheaper than keeping a set of full winters on a spare set of rims. Certainly less hassle.

It's worth pointing out full winters and all seasons tend work with a certain minimum thread depth, particularly in the more extreme conditions, which tends to be greater than what's you'd normally take a summer down to.
So even through they might not wear down to near the minimum legal level, they certainly don't work as well they are getting worn.

So overall value wise when it comes to length of service, they might not be the best choice just for the odd day that you didn't really need to go out.

There also something else worth considering, over confidence.
Yes they might improve grip in poor winter conditions, but there's only so much they can do. It doesn't mean they or you are infallible in these conditions.