Is it correct that only the 4 lower jhanas (rupa jhanas) are useful for cultivating insight (panna) because the meditator's mind is too absorbed when he enters successively the immaterial jhanas (arupa jhanas)?
Entering the 4th jhana (the most focused of the four) is best before doing focused insight practice (vipassana)
The immaterial jhanas are related to some of the Stages of Insight (achieving one can spontaneously lead to the other) but I have never seen it recommended in reference to Vipassana practice. Bavo Lievens was saying in his book that apparently some people who have never cultivated the form jhanas can spontaneously experience some of the formless jhanas! This would make sense why certain drugs (e.g. marijuana, shrooms) lead to states that feel like descriptions of the formless jhanas but do not have the stability and clarity of the form jhanas.
I've actually seen it said in some places that the formless jhanas can be distractive due to their more permanent, expansive nature, making it harder to see the 3 characteristics.
The "form" jhanas on the other hand lead to a centered state of mind which readily allows penetrative clarity into the 3 characteristics, the essence of vipassana and Awakening and disentangling the chain of suffering.
In MN 70, the Buddha taught a seven-fold classification of practitioners. Depending on the various dominant faculties, the immaterial attainments are must-have to some class of disciples (ie. the "Liberated both ways", the "Body witness", etc.)..
That is false. In the Anupadda Sutta (http://www.accesstoinsight.org/tipitaka/mn/mn.111.than.html) the Ven. Sariputta enttered into the various jhanas, arupa jhanas, and the cessation of perception and feeling, and used each of them as a basis for insight. The only difference was that in the case of the last of the arupajhanas (the sphere of neither perception nor nonperception) and the cessation of perception and feeling, the Ven. Sariputta had to withdraw from jhana in order to practice insight.