That's not the point cuz of course he is the 3rd strongest and all… but Oda is overdoing it trolling Sanji or making him looking like an idiot.
What will he do now? Have a fight with Nami's body? No... Zoro and Luffy will get an awesome fight and until Law will be beaten and the effect will go away... Sanji can't do serious stuff.
In Fishmen Island he already had no fight on his own and now it looks bad once again... Ever since TB it is ongoing and that's somehow dissapointing.
Actually, couldn't Sanji have an awesome moment in Nami's body? Like, he can't be responsible for hurting Nami, so he would fight in an unfamiliar body, without getting a single scratch… That is the best that could happen now... But yeah, probably Sanji will be left out again. :/ I'm also kind of dissapointed with that.