Look at the history of F1, we have had such different and crazy cars in the sport historically ... where different designers and engineers took completely different approaches to the cars, designed them with many different things in mind.
Some teams and designers will feel that the car being able to run in any weather will be important, other's will make a car that is good in the dry, but fails in the wet, other's will make cars that are amazing in the warm, other's in the cold..
Now say the races were designed to test all of the above throughout the season, say we had the British GP in April (when it might rain), moved a few other races around ... what would happen? well either you run a compromise car that can do it all (though the car can of course be tuned to specific conditions present during a GP weekend), or you run a car that is only good at some events ... you pick what is the best way to get to the championship.
Right now the biggest fundamental difference between the F1 cars is the diffuser on 3 teams (and the suspension on the Red Bull - but even that is possibly being changed) - yes I know every car is different, but we not have anything totally out of there like some of the things that were around in the past.