It's mot useful for midyears and deciding whether a sub can be in your topm 4 for e.g.
But don't write off a subject after buying a statement of marks and seeing that you didn't get THAT good a score. Know one dude who did Bio in 2009, bought his statement, got 63/75, was quite shattered (he was the best in that year at our school). Almost wrote off Bio, but did SOME study regardless. He ended up getting a 50. (His endofyear score was something ridiculous ike 73.5/75 in a really hard exam)
Quite often it would seem as though your midyear doesn't count THAT much, as your midyear score could give you access to a wide range of scores. I know a guy, for instance, who got 67/72 in chem midyear and ended up getting a 50, and I know a 68.5/72 in chem midyear (and A+ in end of year) and got a 45.