You wouldn't say that if you knew what the word "HujjatUllah" meant. Go ahead and believe he's hiding for millions of years, but don't belittle us by saying the hidden dude is God's divine proof and argument upon His creation. He can't be a proof or an argument if he's absent, a brick wall is more of a divine argument than he is at this point.
As for the ahtiest part, true the angels are hidden but mentioned in the Qur'an, your 12th leader However is hidden but not mentioned in the Qur'an. Also the angels can be hidden since they have no function that requires them to be visible, so the ahtiest won't bother you if you tell him the angels are existent but hidden. On the other hand, if you tell him "We have a divine guide and leader" then you tell him he's hidden. The ahtiest is going to laugh at you since that's equal to saying "We have a holy book that instructs us but is hidden and unavailable." We Muslims believe in a visible Prophet (saw) that came and taught for many years openly his message and then left us a visible book before he departed, that's called a solid belief. You guys believe that some guy who was born in a miraculous way without any signs of pregnancy on his mom, his birth was a secret and nobody witnessed it, he never made a public appearance and hid himself completely for a thousand years, that's a weak empty claim that anyone can in fact ... claim.
I am not forcing my beliefs or views on you.. You may believe what what you believe and I may believe what I believe. There are many things which Non-Muslims ask and from their point of view my belief as a Muslim seems fairy-tale and/or terribly misguided to them.
Before asking me about hidden divine leader atheist would ask about GOD.. atheists laugh at those who have belief in GOD whether Muslims, Christians, Jews, Hindus because they deny the Existence of GOD based on invisibility of GOD. When atheists don't believe in GOD why would they care about HIS divine leader?
Imam Mehdi (a.s) was seen by his (a.s) parents and very close people who were very few. Besides them no one has seen him (a.s) up to this day. I believe in Hidden Hujjat-ULLAH based on what our Imams (a.s) said. If you find it funny or fairytale then it is upto you. I don't find belief in Hidden Hujjat-ULLAH as funny or fairytale. Many of my beliefs are related to unseen matters:
01. Belief in GOD - Unseen i.e. I can't show anyone GOD nor has anyone seen GOD.
02. Belief in Angels - Unseen i.e. I can't show anyone Angels nor has anyone seen Angels besides Prophets (a.s)
03. Belief in Jannah - Unseen i.e. I can't show anyone Jannah nor has anyone seen Jannah besides Prophets (a.s)
04. Belief in Jahanum - Unseen i.e. I can't show anyone Jahanum nor has anyone seen Jahanum besides Prophets (a.s)
05. Resurrection of those in Graves.
06. Judgement Day
07. Our Deeds being Recorded and presented to us on Day of Judgement.
Besides these I also believe in existence of other beings based on Al-Quran e.g. Jinn and Satan. Satan has existence and always leads us to sin but can we show Atheists Satan and how he leads a person to sin?
Al-Quran is a Book from GOD which is used by us Muslims as tangible evidence to prove to Non-Muslims about the Existence of GOD and Prophethood of Mohammed (s.a.w.w).. but are Non-Muslims convinced?
Al-Quran is one Book which every Muslim whether Shia or Sunni believe is a apparent guide for Muslims from GOD yet Muslims themselves are divided into many sects. Why has Apparent Guide of Muslims (i.e. Al-Quran) not been able to stop the Muslims from differing from one another? Why are there Salafis, Deobandis, Brelwi, Wahabbis, Mutazillas, Ahl-e-Hadiths, etc in Ahlul Sunnah when all of them follow the Apparent Guide i.e. Al-Quran?
Then there are Quranists like followers of Rashid Khalifa and there are also Sufis. Even Khawarij Believe in ALLAH (SWT) and believe in Prophethood of Mohammed (s.a.w.w) and believe Al-Quran as their guide. Why couldn't Al-Quran the Apparent Guide had been able to save them from becoming Khawarij?