Did you use the top 50 players ranked by efficiency? Because if so, I would think it would make more sense to use the top 50 players ranked by kills. 
Errr, I'd tend to disagree Leca. The way I see it is, well, someone with a good efficiency will be (presumably) more well rounded than everyone else, and be more likely to show drastic differences in weapon usages due to, well, their usefulness.
That's just my take on it though, I see your point too.
That's the problem though. Most of the top effency players specialise in one weapon and use it more than others.
Amgh - Humans (I guess he doesn't like aliens

)
Johnny - Mara Claw
Aequitas - MD
Meisseli - Chaingun/MD
Warlock - Lasgun
Hamstar - Lasgun
Lecavalier - Dragoon Chomp
Dragonzord - Pulse Rifle
UnnamedPlayer - Lasgun
Net_Giant - Lasgun
these are the weapons that the top ten players use the most, and most of those are by a considerably margin
(most). I would think that If they did use the most usefull weapons for their usefullness, there wouldn't be so much variety. IMO It doesn't show so much usefullness as it does player prefrence.
edit: apparently spell check changes lasgun to lasagna