Well, Approval maps are still archived maps which have a scoreboard. They still count towards your total score, number of plays, hits, combo, replays watched, and level. The only thing the don't count towards is your ranked score.
Ranked maps are a completely different set of ranking material--They only count once per beatmap set, whereas accuracy counts once per difficulty. Accuracy on approved maps shouldn't be that much different from accuracy on ranked maps, so I don't see what the problem is.
A full combo on an approved marathon has an abnormally large score, which is why it doesn't count towards your ranked score, but accuracy is not a total, it's an average, so approved maps should count towards your total accuracy.
Accuracy is supposed to show how accurate you are on all maps. If you only SS or S songs, or fail songs you know you can't 99%, then you are artificially inflating your accuracy, which renders the number useless.