If monks are spreading the true gospel I have no problem with it.
We likely have different ideas about what constitutes spreading the true gospel, particularly as it applies to monastics. No matter..
If monks are living perpetual lives of seclusion, I believe it is unscriptural.
If again you mean perpetual hermits, then you have very little, or very few, to be concerned about.
If you throw out monasticism on this account, you have given up a great deal for something quite small.
My biggest concern is that it be the true gospel.
For example, in Kolo's example, these men had a spiritual experience in which the felt enlightened. So have others. From Mahavira to the Buddha etc., they went away in a time of seclusion to attain enlightenment and came back with some sort of supernatural experience.
But, the experiences that they had fail the Scripture test. If the monks' experience doesn't, then great. But just because there is an experience does not mean it was from God (it could be from self or worse). It must be harmonious with the Word of God.