Getting rid of long discourses from Tipitaka, what could be true intention?
To make a blanket statement that the "DN was not spoken by the Buddha" would not be wise. Obviously, there are certain suttas in the DN that reasonably conform with what the Buddha taught in other suttas as verifiable truth. 

It must be noted the core principle of the Buddha-Dhamma is that is it 'verifiable', namely: sanditthiko akaliko ehipassiko opanayiko paccattam veditabbo vinnuhi. Any sutta that departs from this core principle is open to question. 

It must always be kept in mind that pre-Buddhist Brahmanism was very different to Buddhism however, over time, both Buddhism &amp; Hinduism changed so much that they became largely indistinguishable resulting in Buddhism becoming extinct in India. Many commentators have attributed this to Buddhist efforts to proselytize the religion &amp; the DN has been deemed (for example, by Bhikkhu Sujato; by Bhikkhu Bodhi, page 31 of his SN) to be the major vehicle of proselytization. In other words, it was corrupted Buddhism designed to seduce Brahmans. It is widely acknowledged that: 


  Recent
  scholarship suggests that a distinguishing trait of the Digha Nikaya
  may be that it was "intended for the purpose of propaganda, to attract
  converts to the new religion."




If the DN was composed for the purpose of propaganda, to attract
converts to the new religion, then it is unlikely the Buddha spoke for this purpose, since the Buddha was only interested in converting those with little dust in their eyes (MN 26) and keeping his doctrine pure (AN 2.23, AN 2.25, MN 22, MN 38, etc). 



As for the Brahmajala Sutta itself, it is not the specific sutta best chosen from the DN to make a claim the DN was not spoken by the Buddha because the principles in the Brahmajala Sutta appear sound. It is others suttas best chosen to make such claims, such as the Maha Nidana Sutta, which is obviously a corruption of the Buddha-Dhamma. 

This being said, the Brahmajala Sutta might possibly contain words or doctrines that are not found in any other sutta, such as saññīvādā, antānantikā, amarā­vikkhe­pikā, etc. I think only a competent scholar could make a reasonable hypothesis. If so, it is open to question.

Another peculiar feature of the Brahmajala Sutta is it is a response to a debate between the wanderer Suppiya together with his pupil, the youth Brahmadatta. It is possible but it also seems strange the Buddha would give such a lengthy, pedantic &amp; close to paranoid anal discourse due to the discussions between the wanderer Suppiya together with his pupil, the youth Brahmadatta. 

The paranoia or defensiveness gets even worse, with the morality sections referring to "Gotama" himself abstaining from attending unsuitable shows, games and recreation, etc, to the point of sounding very childish. At least I would expect the Buddha to refer to his Vinaya abstaining from these worldly behaviours rather than himself personally. 

To me, the Brahmajala Sutta sounds like a PhD dissertation rather than the words of a Buddha. That being said, as already posted, the principles in the Brahmajala Sutta are sound. Therefore, there is nothing explicit in the Brahmajala Sutta to deem it not Buddha-Dhamma. 

If one wishes to critique the authenticity of the DN, others suttas are best chosen. 



To conclude &amp; to answer the question, the reason for dismissing DN suttas such as the Maha Nidana Sutta is because they promote wrong understanding of Dependent origination, which is the core dhamma of Buddhism.