nirgun

"Maha Pavitar" - I'm quite surprised at your post. Sikhi is obviously a universal religion. Hair is naturally present on every single person in the world. People only started removing their hair recently because of societal/social pressure. So why would it not be universal for everyone to keep their hair? It's naturally on the bodies of almost all men and women born! Keeping hair would be practical for ALL people.

To everyone else. Even if the Guru Granth Sahib does not explicitly say "keep your hair" - think about it this way - if you become enrolled in a school, none of your textbooks will contain any information about the school UNIFORM. The Guru Granth Sahib Ji is still your TEACHER, but the Khalsa Panth is the school. You need to enter this school and follow the rules of the Khalsa, as ascribed by Dhan Dhan Maharaj Sri Guru Gobind Singh Sahib Jee and his beloved Gursikhs.

Consider this shabad:

paanch singh amrith jo dhaevai thaan ko sir dhhar shhak pun laevai|| apun mil paancho rehith jo bhaakhai thaan ko man mae dhrirr kar raakhai||
(meaning): Offer you head and take the Amrit prepared by five Singhs. The code of conduct (Rehat), which they bless you with, should be enshrined in one's heart.

In all rehat maryadas (akal takht, damdami taksal, akj, etc) keeping all hair is essential. Guru Gobind Singh Ji tells us to follow the rehat given to us by the Panj Pyare. ALSO - the guru granth sahib ji states that you should listen to ANYTHING and EVERYTHING your Guru tells you to do. Jo Gur Kahay Soee Bhal Mano! So that means we should treat ALL Gurus equally. If Guru Gobind Singh Jee indicated that we should keep our hair and told us so take Amrit, we MUST listen to him in order to be Sikhs!

Bhai Desa Singh's rehatnama also states the following:

prathham rehath eaehee hai kehee || paahul main jo singhan kehee ||
(meaning): The most important principle is to forever remember the Rehat given by the Singhs at the initiation ceremony.

We should obviously listen to the rehat given to us by Gursikhs who actually know what they're talking about, like the Beloved Panj

There is another hukam that Shahey ShahnShah Dasmesh Guru Gobind Singh Jee wrote to the sangat of kabul, here is a brief translation:

You should take baptism by the sword, from Five Beloveds
Keep your hair uncut for this is a seal of the Guru
Accept the use of shorts and a sword
Always wear iron Kara on your wrist,
Keep you hair clean and comb it twice a day.
(Guru Gobind Singh)

If you look through the Amrit Keertan book also you will find the following shabads by Guru Gobind Singh -

"Bina Shasthr Kesang Narang Bhed Jano" (Guru Gobind Singh) - Without Kes and Shasters you are considered to be a sheep.

and

"Bina Theg Kesang Dhivo Na Dhidhaarey" (Guru Gobind Singh) Without Tegh (weapons) and hair, do not come to get my darshan.

Hair definitely does assist in doing bhagti, I know that for myself at least. Sometimes Gursikhs have talked to me about how they can feel all of their hairs vibrating with Naam when they do simran :D waheguru!!! I love that feeling when each and every hair on your body is tingling. And you feel so blessed that God himself put each hair on your body. It's a wonderful thing. :)