Allah Taala said in the Quran:
[Shakir 4:82] Do they not then meditate on the Quran? And if it were from any other than Allah, they would have found in it many a discrepancy.
In the book “Akaid al-Imamiya” shia shaykh Muhammad Reda Muzaffar said:
We believe that, just as it is necessary for Allah to send someone as a prophet, so it is also necessary for Him to appoint an Imam. It is necessary that at all times there should be an Imam to represent the prophet, and that he should perform the duties of the prophet, such as guiding the people, and showing them the way of goodness and prosperity in this world and the next. He ought also to hold the highest position as a public authority in all aspects of people’s lives, so that he may cause Justice to increase among them and eliminate enmity and oppression from between them. The Imamate is therefore a continuation of the prophethood, and the reasoning which proves the former’s necessity is the same as that which proves the latter’s.
See. Chapter 23: Doctrine of the Imamate.
So presence and availability of Imam is necessity in shia faith. He should judge between peoples, establish justice, help oppressed, and punish oppressors. You see the question?
Shaykh of shias, allama Baqir Sharif al-Qurashi in his book “The life of Imam al-Mahdi” said:
The major occultation began after the death of Ali bin Muhammad Samari in 328 A.H.
See: Chapter “The Major Occultation”.
Now is already 1431 year by hijra. Obviously 12-th imam DIDN’T judged between people, DIDN’T help oppressed, DIDN’T punish oppressors during 1103 years!!!
Shias claim that it was necessary for prophet (sallalahu alaihi wa ala alihi wa sallam) to APPOINT successor, which should be masoom (sinless), so he can represent prophet (sallalahu alaihi wa ala alihi wa sallam). They claim that it’s important for Moslem nation to have their own masoom Imam. WHERE IS HE? What has he done f0r Islamic nation during these 1103 years?!! A single deed! NOTHING!