Obviously, everybody wants their robot to quickly accelerate and move around the field. But if you make the robot too light and gear your motors too high, you will be left with very little ability to push back against other robots.
If playing defense and counter-defense is a part of your strategy, then another important consideration is how much pushing power you can get out of your robot’s weight. This conversion is determined by the friction coefficient of the wheels that you are using.
For example, for 4" omnis it is about 1.3, which means that 15 lb robot on 4" omni wheels could generate 19.5 traction in respect to the ground, which is just slightly more than max 18 in*lbf force generated by 4 V5 motors with 200 rpm cartridges at 2" radius (more details here and here).