Each class fills distinctly different roles within the game, if you play in a proper squad you need all 3 of the 'assault' classes to maximise your efficiency, support to lay down heavy fire and close down lanes of attack for the enemy, the engi deals with any vehicle threats which can seriously outmatch a squad if left to their own devices and you need the assault to keep people in the game, reviving and medkits are a quicker solution to respawning! The last slot can be anything, recon has its value as a long range spotter but imo at least until he unlocks the mortar he's the weak link as far as im concerned (recon is my favourite class

), you'd be better off in most cases with another assault or engi in most encounters.
I'm quite comfortable playing any class so I usually just have a look at what the team us lacking before I spawn and go for that, you maximise your points that way too, everyone's a winner