The "rubber ring", as you describe it, is a shock absorber that is, itself, supposed to carry the weight of the engine and buffer vibration/shock at it's points of support. Without, the shear pin/mounting bolt will suffer and surely fail and, undoubtedly, at the most awkward time, when you need and are working the engine hard. The mount should be replaced and the others inspected closely. If they are of the same vintage, they may need replacement as well.One of the motor mounts on my VW Pathfinder in a 37 foot Truant Sailboat is missing the rubber ring. How big of a deal is this? As far I can tell there is no excessive vibration.