That means they are running the MP4/24 like that to give Paffet a baseline before they put him out in the B spec car that finished the year. Get a feel for the improvements to Germany, then give him the car from then on to get a feel for the car, then run him in the car that finished the year.
The rest of the teams are probably using the cars that finished the year. Id recon that some of the 2010 parts and tyres will come out tomorrow and Thursday, get the driver a baseline to work with, then improve and remove from there.