At this point I don't even know what the end game is supposed to be in this game. Is it reaching level 40? Is it gym control? Is it catching them all?
Even if you are going to attack with pidgey you'll probably defeat the legendary, as quantity is more important than quality these days.
If you are preparing for legendaries and you enjoy having a variaty in line ups, you should do it. However, I bet that dragonite is gonna do just as good against legendaries as pinsir.