Under what circumstances would you purchase real estate with only a Quiet Title? I mean how credible is Quiet Title?

I think you might be mixing up terms. Are you talking about purchasing property with a quit claim deed? Or are you talking about purchasing property where there has been a quiet title action to confirm proper ownership?

If you are talking about purchasing property with a quit claim deed, then as long as a title insurance company will issue a title insurance policy then I would have no problem with the purchase. The title insurance policy will insure that title is clear and they will defend and reimburse the owner if someone comes along with a better claim to the property.

If you are talking about purchasing property that has been the subject of a quiet title action, then again I would have no problem with the purchase as long as the title insurance company will insure the title. And with a quiet title action it is usually easier to get title insurance since a court has determined who has the best claim to the title.