I’m not discouraging people from taking engineers and building stuff, in fact I hope more people would do so.
I’m just stating that the game does not reward building structures (especially rallies) enough.
Taking engineers and building for the team, as of now, is very much an altruistic behavior that benifits the team but not as much the player himself.
As a prime example, “The best engineer” award goes to the engineer who has the most kills, not the one who contributed to most to the team, like, whose rally was used most.
If the rewards " w e r e " attractive enough, there will very surely be enough people taking engineers and building stuff. However, I don’t think the developers want everybody to camp behind caps and hammer away, so this is unlikely to happen.