Overall offensive play is more fun than defensive play. The benefits of defensive play should decrease the further along the game gets.
In terms of costs/benefits a team that focuses overmuch on early offensive play should lose out later in the game. The cost of early defensive play (!not camping) is that you might lose a few structures but you will have a solid foundation for the rest of the game. If on the other hand a team stays in turtle mode for too long the costs/benefits should change. They are now losing many structures and no longer gaining much of a foundation for later game stages. The weapons available should reflect that. Letting your opponent anywhere near your base late in the game should be very costly and quickly snowball.
