The point is that a pure devotee can't be seen with material eyes and material logic. Just like Lord Krishna can't be seen with material eyes, so also His pure devotees. You need spiritual vision to appreciate God, His abode, His name and His devotees. Thus Bhakti Vinod Thakur, one of the greatest spiritual teachers of all times quotes the following verse from the Mahabharata which is generally sung before honoring prasadam.
svalpa-punya vatam rajan,
visvaso naiva jayate.
"Those who don't have sufficient pious credits cannot develop faith in Maha-prasadam, Lord Govinda, Holyname, Brahmanas and the Vaishnavas."
Sometimes Lord Krishna puts an apparent curtain of imperfection in front of our eyes, so that only those with faith can appreciate His devotees. As the saying goes in English, "It takes one to know one." That means to appreciate a pure devotee, we need to ourselves be on the spiritual platform. In fact it's interesting to see people find defects and imperfections even in the All-perfect Supreme Lord Krishna. How then, will their mind spare The Lord's pure devotees? The defect is not in The Lord or His dear devotees but in our eyes.
Following are few examples of devotees who apparently had defects but are still considered pure devotees.
1) Garuda is considered to be the most intimate devotee of the Lord, His personal carrier but he eats fishes and snakes.
2) Bhisma is considered to be one of the 12 best devotees of The Lord but he fought on the side of the Kauravas and against Lord Krishna and His devotees.
3) Devotees are known to have control over their anger and hunger, but Bhima would eat voraciously and was quite hot tempered.
4) Tukaram Maharaj, although fully immersed in glorifying Vitthal didn't take care of his family that well.
5) Probably, the most classic example of a defective pure devotee is Vrtrasura. He was the King of demons and was fighting and killing the demigods but he is considered to be the topmost devotee. His devotion becomes clear in his consciousness at the time of his death.
We could quote more examples but I am sure this much is enough to completely short-circuit our "so called" logic. Just like Lord Krishna is inconceivable, so also His pure devotees. They are beyond the reach of our mundane mind and senses. That is why it is said - vaishnavera kriya-mudra vijneha na bujhaya: even the most learned man cannot understand the activities of a Vaishnava.
So, the question may arise, what is the criteria to identify a pure devotee? Lord Krishna Himself answers this question in the Bhagvad-gita 9.13 by defining who a pure devotee or a mahatma is. He says, mahatmanas tu mam partha, daivim prakrtim asritah. "A great soul or a pure devotee is one who has taken shelter of the divine nature". In other words a pure devotee is one who has taken full shelter of Lord Krishna and the process of Bhakti. That is the primary criteria, the rest is secondary.
In summary a person is considered a pure devotee not from our viewpoint but from Lord Krishna's viewpoint.
The safest path is not to judge any devotee. Best is to see everyone as a pure devotee or at least as a "pure devotee in the making". During the initial days of ISKCON, someone asked Srila Prabhupada, "How many pure devotees are there in ISKCON?". Srila Prabhupada asked one of His disciple, "How many disciples do we have?". The disciple replied, "We have 72 devotees all over the world." Prabhupada turned to the person asking the question and told him, "Right now, we have 72 pure devotees." Srila Prabhupada saw all of them as "prospective pure devotees".
Just like if someone is standing under a shower, it is but understood that very soon he will become clean. The question is never "if he or she will be pure", the question is only "when". Till then let us bask in the company of these pure devotees and be grateful for thier association because we really don't have any clue as to "who is who" and "who is where" on the spiritual path. And if we come to know, we may be in for a big shock.