Why is there is persistent and perpetual attack on the Shresthas from all sides regarding their history and heritage? I ALWAYS see the same argument being made - that Shresthas are not proper Newars, that they do not represent Newari culture, yada yada.

If these people actually care to read a few books, they will realize that Shresthas obviously are an inseparable part of the larger Newar community. As the traditional representative of the Hindu Kshatriya (warrior, administering) varna of Nepal Mandal / Nepal Valley, Shresthas have no doubt have been the most dominant and most privileged groups of Nepal. They included among them the rich landholding clans, they included among them the Thakuju (Thakur-jyu) royal family and nobility (Malla, Pradhan, Pradhananga, Rajlawat, etc.) they included among them the highest non-Brahmin priests (Joshi, Karmacharya, Gurubacharya), they included among them the highest administrators of the land (Rajbhandari, Amatya, Bhadel, Mulmi, etc.). Ethnically, Shresthas are mostly derivative and hodge-podge of Licchhavi and Malla mixed with a good sprinkling of Kayastha, Rajput, Karnat (Maithili), Varma-Thakuri (not to be confused with Khas-Thakuri), Brahmaputra Kshetriya and few prominent Vaishya heritage, and also no doubt due to geographic proximity to Mongoloid influences.

Words like 'Shrestha' or 'Pradhan' were already introduced in the 17th century since Pratap Malla's time, although the medieval name for these were "Bharo' and "Pradhan Mahapatra (Pamah)' respectively in the olden days. To say Pradhan worked as rice distributor that also without any basis or justifiable proof is just absurd. Sure they were last of the major migrants to enter Nepal, and they certainly came to Nepal Valley much later than other Newar groups, but almost all the other groups too have a history of migration to Nepal Valley either from India or Tibet. And these later migrants adopted and continued the language and culture and festivals of the already existing (and majority) Jyapus and other groups. 

And what most people may not know is that the original meaning of the word Newar almost always denoted only to those upper caste Shresthas, the Chhathari Shresthas. Historically those Chhathari Shresthas (not all but prominent ones like Rajbhandari, Rathaur, Joshi, Vaidya, Karmacharya, etc. and few other Newar castes like the Naya/Khadgi butchers) are written in the genealogical records to have come with a Maithili-origin king in the 1400s to escape the Muslim emperors. That king originally came from the Neyaar region of the Malabar coast of Maharastra/Kerala. Hence, it is believed (although by no means proved) that the Neyaar > Newaar name stuck and begun to be used to those immigrants - who are largely part of the Chhathari Shrestha caste among Newars. *And the Naya/Khadgis till today claim to be descendants of the Nayars of Kerala. Google it up to know who they are. If you go to the predominantly Jyapu villages like Khokana, Godavari, Panauti, etc. the older generation farmers there will equate the term Newar to the Shresthas and not necessarily to the whole community.

And on the importance of Jyapu as the bearers of all the culture of Newars, no doubt. Jyapus no doubt have carried the duty as patrons of all the Valley jatras and socio-religious events, and almost all the calendar functions of the year.