Why using Diamonds to unlock the "Score Card" is a great idea.
Unlocking the score card is a great way to monitor your career progress across each or the MotoGP circuits. Not only will you gain useful information which you can compare against the real race lap times, but unlocking the "score card" will give you access to everyone else's score card.
This is really useful as you can see how your friends are doing on each track and begin to see how people amass the scores they do.
The COMBO is the best driver of score, so having a really high combo is essential to optimizing your score on each track. When you get a PERFECT you will see your combo goes up by 3, so someone who has a really strong track record of getting perfects on a circuit will likely have a really high COMBO and makes very good use of the COMBO Insurance in the Last Chance Performance Boosts.