Well, aspiring Bodhisattvas should be well-versed in the three vehicles, since different beings are receptive to different vehicles. However, one should be especially careful not to cultivate the supramundane paths of the Sravaka and Pratyekabuddha paths too deeply, since attaining the fruition of these paths would result in irreversibility and cut off the possibility of attaining Buddhahood. So the key thing is to, yes, study these paths by all means, but not attain their final fruit. Nagarjuna states in the Bodhisambhara-sastra:
Thus it is that those bodhisattvas,
When striving for the realization of bodhi.
Should not rest in their practice of vigor,
For they have shouldered such a heavy burden.
Until one develops the great compassion and the patiences,
Even though he may have gained irreversibility,
The Bodhisattva is still subject to a form of "dying"
Occuring through the arising of negligence.
The grounds of the Sravakas or the Pratyekabuddhas,
If entered, constitute "death" for him
Because he would thereby sever the roots
Of the Bodhisattva's understanding and awareness.
At the prospect of failling into the hell-realms.
The bodhisattva would not be struck with fright.
The grounds of the Sravakas and the Pratyekabuddhas
Do provoke great terror in him.
It is not the case that falling into the hell realms
Would create an ultimate obstacle to bodhi.
If one fell onto the grounds of the Sravakas or Pratyekabuddhs,
That would create an ultimate obstacle.
Just as is said of one who loves long life
That he is frightened at the prospect of being beheaded,
So too the grounds of the Sravakas and Pratyekabuddhas
Should provoke in one this very sort of fear.
It's not an easy thing, of course, to be expected to be well-versed in all three vehicles, and yet avoid slipping into arhatship. It is like an archer keeping his arrows aloft in the air by firing each one upwards perfectly behind the other (another analogy used by Nagarjuna).
Interesting thing: in the Ratnakuta collection, Manjushri mentions that he has liberated countless beings during the dark eons (where the teachings of the previous Buddha have vanished, and the next one has not arrived yet) by assuming the forms of Pratyekabuddhas and demonstrating entry into Nirvana. But due to his vows, he did not actually enter into extinction.
Yes, my point is that attaining arhatship or
the first bhumi are likely to be fairly remote possibilities at this point.....where gaining good overall knowledge of the dharma in all its traditions should be encouraged not discouraged.
I suppose your point is that intentions even in the early stages of the path matter; and sure, they do. But how do you know which path to follow and what intentions to cultivate without inquiring into and listening to many teachings?
The thought that someone will be spiritually harmed by encountering the Theravada is scandalous and should be immediately repudiated.
Seek freedom and become captive of your desires. Seek discipline and find your liberty.