I could not imagine me doing this stuff without a physics engine on hand. I remember, not so long ago, the pain of collision detection using the traditonal software methods........lots of maths,vertices,vectors and matrices.......AND valuable TIME.
Now, we can fire a projectile in the air without any need for calculating its trajectory.If some object is in its path, so what........the collision will take care of itself! In my 1st person game, I am free to bump against walls,climb over obstacles,pick up and interact with objects, enable realistically moving machinery.........all with the aid of a GOOD physics engine.However, if the physics engine is NOT all that good, then one will get unrealistic performance and time will be spent trying out all kinds of workarounds to achieve the effect required.
An example of collision between two rigidbodies can be seen in the Eureka project, where the spring balance hook has to catch and hold onto the handle of the block "bodies". In the case of 3D Rad, if you plan the distribution of your spherical collision bodies and mass bodies correctly, then everything should go according to plan.