I have wavered on this over the years, as I have in so many other things, but my present thinking is that all Vajrayāna practitioners of whatever stripe need a solid grounding in Hinayāna and Mahāyāna paths.
I think this a very sound thought. I recall Dzongsar Khyentse Rinpoche talking about too many Vajrayāna practitioners sitting around reading shastras but rarely any sutras and that this was not a good thing.
A good result of a thorough grounding for practitioners would perhaps be a better ability to make decisions about a lama or teacher and whether or not they were upholding a family lineage, were a true tulku, or whether they had the merit to be a teacher. To use a metaphor, if you want to go hear someone teach you about quantum physics it might be a good idea to first know whether or not they know the principles of a lever or even how to use one.