Urgyen Tulku says in his book Repeating the Words of Buddha that when somebody attains the rainbow body then simultaneously 2000 people attain enlightenment.
I have heard something similar by one highly respected lama. He said there was a special liberating effect caused by attaining rainbow body that had a profound liberating effect on other sentient beings. I also got the feeling he was saying this not just as some casual information, but as an inspiration to aspire to that path of practice, as it is also highly suitable for those who have the capacity to achieve high realization but do not have much karmic connection for outer bodhisattva activity in this lifetime.
Pema Rigdzin wrote:
Maybe I've missed something here in Narraboth's answer but I still don't see the use of dissolving the physical body into rainbow lights besides its obvious miraculous nature that can inspire faith. Also, if we go into the land of wonders, bodhisattvas are capable of all kinds of magic transforming themselves into virtually anything, including buddhas. And that ability is available to a large number of gods, demons and yogis too. So, again, what is so special about the rainbow body that it can be an argument for the superiority of dzogchen?
Honestly there's much more to the significance of rainbow body that is only going to be explained in the context of teachings on togal which explain how wisdom and the 5 elements have yet to be totally liberated in clear light before the fruition of that level of practice (the 4th of the "four visions"). Dzogchen mengagde teaches that even the fruition of trekchod is not quite complete and does not result in the maximum
ability to benefit beings (although one's own benefit is completely achieved and a very immense benefit to others is as well, obviously). For people like us, though, I think Narraboth's example of being like ants trying to estimate which skyscraper above us is taller is quite appropriate, even if it is interesting to talk about.
I think there is something quite significant to the ja lü
that Pema Rigdzin points to here, that needs more elucidation to understand its profoundity.