Thanks for the replys guys.

Looked into in some more and have come to the following points:

1: Slicks are made of a much softer rubber then road tyres so once warmed up provide a much more sticky contact area to the road then a hard, cold road tyre.

2: As they don't have all the groves on the tread like Alex said, they have a larger contact area as well as the above contributiing factor.

3: Also as Nilrem said, they are made of a much more higher end rubber and don't have to last ten thousand miles under all condtions like a normal road car tyre. Similar idea applies to using them in the wet as well I gather.

Another quick question though, why would normal road tyres overheat if tyre warmers were used? Is it because they aren't upto the the temperature loadings? I only used the warmers on balk tyres as a case point, I wouldn't do it in reality.