It's not only the weather that can be an issue in December, the lack of daylight hours can also be.
As you'll see from this site
http://www.timeanddate.com/sun/uk/carlisle?month=12&year=2014 the number of hours of daylight for Carlisle (the nearest city to Scafell Pike) in December is just over 7 hours.
The route from Seathwaite to Scafell Pike and back will take on average 7 hours, that's in good weather and done by someone knowing the route. The shortest route, the Brown Tongue route up from Wasdale, will take about 4 1/2 hours. On top of these times you also have to take travelling to and from the start of the walk into consideration.
So, the answer to your question about whether going up Scafell Pike is a daft thing to do in December, all depends on your fitness level, your knowledge of the route, the route you take, the weather, and also the conditions underfoot.