I think you're right in that alot of them are situational. BUT I think that was Riot's goal. They want to give us options for every fight we're in. So, if we have a specific comp we're against, or a specific strategy we're using, we have mastery options for that. So, no, it's no longer the mindless 9-21-0 or 21-9-0 builds any more. Each game you have to decide what you want/need.
Also, the numbers you're complaining about, as many people have noted, these are bloody masteries. They aren't going to be gamebreaking numbers. They are designed to work with runes/items you buy. They themselves are not enough to do anything on their own. You're right. They weren't designed to though. They are small numbers that add up but do make a difference.
I will concede that a couple of them are lackluster to others, but it's not nearly as bad as you say.