Vaheguru Ji Ka Khalsa Vaheguru Ji Ki Fateh! Recently I have noticed that many people have questioned the importance of Dasam Bani, There was even an event that some people plotted to amend the sacred Amrit Sanchar ceremony, due to their intense feeling that Dasam Granth is not important. I will hopefully, (Guru Sahib's Kirpa is the only way I could) explain why Dasam Granth bani is very important for anyone who calls himself a Sikh.
First, through Dasam bani we find the need to see the greatness of Dhan Dhan Sri Guru Nanak Dev Ji's message, all over Dasam Granth is the importance of how many have lost with their own Haumai, and that they couldn't escape death and couldn't free anyone. This is very important because of how different Sikhi is compared to the various faiths, that nobody goes through Sach Khand, (highest spiritual state) without both Gur-Prassad and Naam.
Second, Dasam Granth provides an opportunity to understand the need for Bir-Ras, (Gurbani has both Shakti and Bhagti, Bir-Ras is Shakti-mostly) even though Dhan Dhan Sri Guru Granth Sahib Ji has both Shakti and Bhagti, it's main focus is bringing Bhagti; while, even though Dasam Granth has both as well Shakti and Bhagti, it's main focus is Shakti. This is important because it builds the importance of the Khalsa to stand-up for righteousness and the defenseless.
Finally, Dasam Granth from the beginning only praises Vaheguru, (for some this is hard to understand because sadly, they don't fully understand the Chaubis Avtar bani in Dasam Granth), the Chaubis Avtar greatly mentions how even someone whom the world calls great they still fall short in the house of Vaheguru and Dhan Dhan Sri Guru Nanak Dev Ji. This is very important to understand, as Guru Sahib has never advocated the greatness of these beings, but rather Vaheguru, in Chand-Di-Vaar, a bani many anti-Dasam Granth advocates use, (they claim that Dhan Dhan Sri Guru Gobind Singh Ji couldn't have wrote that as there were various "positive things" of Durga), the bani starts by saying when he looks at Bhaguti, (as in Shastar, Kirpan), he remembers Dhan Dhan Sri Guru Nanak Dev Ji, and then says the names of all the Gurus up to Dhan Dhan Sri Guru Tegh Bahadur. If Chand-Di-Vaar advocates anything, it respects Shastar, (Kirpan).
May Vaheguru please bless us with the understanding of what your true message is, we may never understand the message, but let us never attempt to disrespect what you say, in whatever form. May Vaheguru bless us with the understanding of the true essence of meaning of any form of Gurbani, instead of leaving any of us astray. Vaheguru Ji Ka Khalsa Vaheguru Ji Ki Fateh! (Below is a Punjabi Steek, with meaning in Punjabi for those who know Punjabi as well as a English translation of Jaap Sahib)