Why are we talking about this, it's the mid-Atlantic, and we have all season tires not summer tires? I don't need snow tires because it doesn't snow that much here. Plus, I have all-wheel drive (or four-wheel drive), so it doesn't matter.*
Well...that's just it. "Snow Tires" and "Summer Tires" are really misnomers. For most applications, what people call "Snow" Tires are generally "Winter" Tires. And "Summers" are actually three-season tires - since they outperform all-season tires in all three of those seasons, whether wet or dry. Their rubber compounds are optimized to maximize grip in these situations, at the expense of grip when temperatures get chilly.
By contrast, all-season tires sacrifice overall performance in everyseason, so that they can still be pliable and maintain some level of grip when it gets cold.
Come on, are we really doing physics again? Well, sort of - but that's because we need to understand the physical properties of the rubber. Every tire has an optimal temperature range where it is pliable but not "greasy". Cold summer tires are essentially rock hard in winter temps which compromises their grip. Winter tires used at high temperatures start to become too soft and lose grip for that reason, and they also wear much more quickly. But this is why they shine when the weather gets cold - whether or not it's snowing, as you can see in this great review that Consumer Reports prepared several years ago.
Over and above this, there is also a compromise between treadlife and grip. Harder tires last longer but have less grip - whereas softer tires have more grip, but that also means they tend to wear down more quickly. Watch any recent Formula One race if you want to get an idea of how this plays out in practice.
So do you need winter tires or not? Most enthusiasts would say you do. But then we're also willing to drop an additional $1,200-$2,500 for a spare set of tires and wheels that we'll use for only about 3-5 months out of the year.
Back to the original question, if I have all seasons, do I need to get a set of winters? Most likely not. Except in the most extreme conditions - in which you'll probably not be driving anyway - you can probably do just fine with the all seasons that are on your car.
If your car were equipped with summer tires, then you will almost assuredly need winters when it gets cold. Think that doesn't apply to you? Maybe not, but summer tires are actually more common than you might realize - coming as original equipment on Mini Coopers and most luxury-performance brands.
Additionally, because of the rubber chemistry we saw above, winter tires have more grip than even all-seasons even when the roadways are bone dry.
Changing tires twice (or more) per year adds complications. Who has time for that? Plus, where do you store them? Well, if you have the time, inclination, and storage space, you can change your own and save the cost and time of taking your car to someone else to do it for you.
At a minimum, you'll need a quality jack, jack stands, torque wrench, and chocks to block the opposite wheel - as well as a flat surface to do the work. For safety, you definitely need to make sure your equipment is good. For example, you don't want to use the jack stands that were recalled, after they replaced the other ones that had already been recalled.
So what do we think? That you should run winter tires in the winter if you can. That you most likely don't need that fancy, expensive, and weight-adding AWD system to get enough grip on winter here in the mid-Atlantic. And that - like us - you're probably going to do whatever it is that you want to do anyway :)
Me? I run winter tires when it gets below 45 F, on a performance car with all-wheel drive. I also recently switched to performance all-seasons so that I have more flexibility when to change on and off than dedicated summer tires - i.e., the temperature can get lower before I absolutely have to switch. The experiment is going well so far after two cycles of switching from the performance all seasons to winters and back.
Oh, and can you run winter tires on the rims that already come with your car? Sure. But you'll also get more wear on the tires having to put them on and off twice a year, you risk damage to the rims when this is done - and you'll certainly also have to pay an installer to do it for you.
What about -1 and -2? I've already nattered on enough - so suffice it to say, you want narrow tires to dig in to snow and ice (more pounds per square inch than wider tires). This mean you can get better winter performance - and also save a few bucks - by going with a narrower tire on a 1" or 2" smaller wheel (-1 and -2, respectively), if your car will accommodate it.
To answer the final question on whether winter tires make a difference on all-wheel drive cars, please check out this video on Tire Rack.
Back to the physics of the situation, an all-wheel drive car on all-seasons will accelerate more capably than a two-wheel drive version. But, then you have a heavier vehicle with sub-optimal grip that you still need to stop and turn. In this situation, you want to have the highest level of grip available to you, regardless of the number of wheels that drive your car. Personally, I still put winters on my AWD car. And for a new vehicle purchase, I might even recommend a two-wheel drive car with winters before recommending an AWD. This is due to significant cost and weight savings, and corresponding improvement in fuel economy.