To put the safety improvements of winter tyres at below 7C into some perspective, their best improvement on stopping distance is obviously on snow and ice and according to most figures I've seen you get about a 20% improvement which is pretty good
http://www.conti-online.com/generato...winter_en.html
At 4/5C, which is my main gripe as lots of manufacturers justify the cost by looking at average temperatures and suggesting anything below 7C makes winter tyres a necessity, the improvement on wet roads is visible but now only about 10%, e.g. Contis below
http://www.whatcar.com/car-news/wint...ditions/259257
So you could argue it's worth getting them even if you get hardly any snow for this 10% improvement. But then compare this to the % improvement you get from using a brand new tyre over a slightly worn tyre and then against a legal minimum tread tyre.
The difference between the stopping distance on a new tyre and at 3mm (the point at which the manufacturer recommends you replace it) is 22%. If you take it down to the UK legal limit it's 53%!! So the 10% you get at 4C above is nothing in comparison to using a new tyre over a slightly worn one.
And re. driving safely, your stopping distance is most affected by your speed and you can easily decrease your stopping distance much more than you ever could by using different tyres by simply reducing your speed in treacherous conditions. You could argue some people on winters go too fast because they think they have more of a safety net
In summary and IMHO, get winter tyres 100% (although all seasons may suffice for some) if you get a lot of snow and freezing temperatures but forget the 7C threshold as a justification. 7C is simply the point at which the stopping distance graph starts to show marginal improvement - albeit only on wet roads. The statistics produced by the tyre manufacturers themselves don't provide any meaningful improvement at 7C.
Feel free to shout me down and provide some different statistics