Its important to understand that revelation in Islam is not just one thing, (the Quran), but both the Quran and Sunnah of our beloved Messenger Muhammad sallallaahu alayhi'wa'salam.
Allah says in Surah al Baqarah
151. Similarly (to complete My Blessings on you) We have sent among you a Messenger (Muhammad SAW) of your own, reciting to you Our Verses (the Qur'ân) and sanctifying you, and teaching you the Book (the Qur'ân) and the Hikmah (i.e. Sunnah, Islâmic laws and Fiqh - jurisprudence), and teaching you that which you used not to know.
So Allah taught the Prophet both the Qur'an and the hikmah (i.e., the Sunnah). So revelation was two things.
As Muslims we are obliged to obey the Messenger. Allah says
59. O you who believe! Obey Allâh and obey the Messenger (Muhammad SAW), and those of you (Muslims) who are in authority. (And) if you differ in anything amongst yourselves, refer it to Allâh and His Messenger (SAW), if you believe in Allâh and in the Last Day. That is better and more suitable for final determination. - a'Nisaa:59
7. ...And whatsoever the Messenger (Muhammad SAW) gives you, take it, and whatsoever he forbids you, abstain (from it)[] , and fear Allâh. Verily, Allâh is Severe in punishment. - Surah Hashr: 7
31. Say (O Muhammad SAW to mankind): "If you (really) love Allâh then follow me (i.e. accept Islâmic Monotheism, follow the Qur'ân and the Sunnah), Allâh will love you and forgive you of your sins. And Allâh is Oft-Forgiving, Most Merciful." - ali Imran: 31
80. He who obeys the Messenger (Muhammad SAW), has indeed obeyed Allâh, but he who turns away, then we have not sent you (O Muhammad SAW) as a watcher over them. - a'Nisaa: 80
So through these numerous ayat we can see that following the Sunnah is an obligation upon us as Muslims. The Qur'an tells us to pray, fast, pay charity, go to hajj, but how? How do we know these things without the Sunnah of the Messenger sallallaahu alayhi'wa'salam?
The only way to access all of these teachings are through the hadeeth. We must also realize that the Qur'an IS a hadeeth. With the highest chain of narration. So those who reject hadeeth are themselves rejecting the Qur'an. The term hadeeth is mentioned 23 times in the Qur'an and both Allah and His Messenger refer to the Qur'an as a hadeeth.
Allah says in Surah al Qalam
44. Then leave Me Alone with such as belie this hadeeth
And from the hadeeth related by Jaabir ibn Abdillah that the Messenger of Allah sallallaahu alayhi'wa'salam said
“Indeed, the best form of communication is the Book of Allaah…” - Saheeh Muslim, Musnad Imam Ahmad
The problem with many is that they understand revelation to only be the Qur'an but dont even understand how the Qur'an was revealed and documented. Our deen is comprised of BOTH the Qur'an and Sunnah.
Hope this helps, wallaahu' 3lam