Well that’s one of the easiest things to test, isn’t it? Just take a tank, don’t do any damage and observe the beacon’s damage.
They manage to finish off the weaker mobs. But the damage is not high at all. For instance, tier1 beacon dmg is 1.5k-4.5k to 4 targets. It is not scaled by any player bonuses and can only take advantage of elemental RR applied to the target.
Compare this to a regular leg. item proc, chain lightning: 126-405 against 5 targets. After some 30x usual dmg multiplier for an average elemental build it translates into 3.8k-12.2k dmg against 5 targets.