Instead of doing 'good' such teaching does harm. Without Guru one can also proceed on the royal path of self-realization. But then the journey would be full of obstacles and pitfalls. It would take long time to reach the destination. This sadhana is like trying to cross the ocean in a small boat.
Guru is like a huge steamer to which one ties one's boat and relaxes.
That huge steamer is sure to take the disciple to other shore. The dangers are minimized and the delay cut down. When the disciple has the vision of Ishta - his chosen Ideal, through Guru's grace, he finds the Guru merging in Him. There is your Ishta. Saying this the Guru merges in the Ishta.