How do tyre compounds work?

One of the things I don't quite understand about MotoGP is the use of the tyre compounds. I know the softer the tyre the faster speeds it allows but the quicker it wears out. My intuition tells me the softest tyres would be used for qualifying, leaving medium and hard ones for the race. However, this weekend in Le Mans almost all the field were on Soft-Soft combinations. Wouldn't have the tyres ran out by the end? In Qatar lots of the guys were on Medium-Hard. Why this difference if both races are about the same length?

One of the things I don't quite understand about MotoGP is the use of the tyre compounds. I know the softer the tyre the faster speeds it allows but the quicker it wears out. My intuition tells me the softest tyres would be used for qualifying, leaving medium and hard ones for the race. However, this weekend in Le Mans almost all the field were on Soft-Soft combinations. Wouldn't have the tyres ran out by the end? In Qatar lots of the guys were on Medium-Hard. Why this difference if both races are about the same length? http://ifttt.com/images/no_image_card.png