"not necessary" was Prabupadas instruction for George Harrison, not you or people in general coming to Krishna Counsciousness at all.
Prabupada followed the system of Guru Parampara (and stressed it's importance over and over again) as is followed not only in Gaudiya Vaishnavism, but also in every other tradition in India be it Shiava, Vaishnava, Smarta etc.
Prabupada often quoted the famous verse from Padma purana:
sampradayavihina ye mantras te nisphala matah: "Mantras which are not received in sampradaya are considered fruitless.Unless one is initiated by a bona-fide spiritual master in the disciplic succession, the mantra he might have received is without any effect."
Now we know that chanting Hari nam without diksha will bring one to a certain point, give some taste indeed. But initiation from a proper preceptor is without doubt of the upmost importance and to minimize that is making up your own philosophy and against the teachings of the Gaudiya Sampradaya and certainly not in accordance with the teachings of the Gosvamis, in particular Sanatan Gosvami and Hari Bhakti vilasa.
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