Slippage of the outer ring (in relation to the inner hub) is not that uncommon and the only thing it affects would be if you are trying to adjust ignition timing solely off of the balancer mark (which I don't use that method).
In extreme cases, the outer ring can come off of the hub and rattle against the crank pulley. The engine will still start and run - but without that balancer ring in-place you could end up with a broken crankshaft.
You are correct about the engine balancing being affected - but not that much an issue on a 318. 360's (all) and late 340/400/440's (with a cast crankshaft) will be more of an issue with vibration - but even without the outer ring in place, that will not cause a engine misfire.
The harmonic balancer is a misunderstood part. The thicker/heaver the balancer (think 426 Hemi) is, the far better for the engine. A car will run without a balancer or with a skinny/thin balancer - but you will break the crankshaft in little time. It has to do firing pulses causing a rapid acceleration of the crankshaft then rapid deceleration with a piston is moving up for compression stroke. Think about playing with a wooden (kids) toy that has square (or triangle) wheels. The axle speed will not be constant when a square wheel rotates on the ground. The 426 Hemi's, 361/413 industrial engines and many of older 440 Imperial's use the thick/heavy balancer (the Hemi version is different) - but the crank pulley(s) are different because of spacing. Sense it costs more to make a heavy balancer - is the reason not a lot were made.
The thickness of the rubber and distance of rubber ring from center of crankshaft allows of the correct amount of deflection for crankshaft acceleration/deceleration. The problem is the rubber wasn't bonded to the metal like the newer balancers are - so over time, the outer ring slips (rotational).
When inspecting a balancer (before installing one) be sure to check the rubber for major cracks or to see if it is "oozing" out of its grove. If so, discard yours and get another balancer. I "think" pretty much all new balancers made in last 25 years use bonded rubber. I wouldn't go through the trouble to change a balancer that has major cracks in rubber (or oozing out) - unless the balancer is already removed for something else.
I have only seen three balancer rings come off. One was on a 400k mile 318 and others were on non-Mopar's.
Somewhere, deep in my garage, I have a mopar balancer (360, I think) that is missing a 3 inch (7.5 cm) stretch of rubber (between the rings) and you can not get the outer ring to pull off of it by using your hands. I can move it using a pry bar, though. It is one of odd things I ran across and kept - for some odd reason).
BudW