In most of these OSAS churches you can have pizza and a beer with the pastor.
I am going to make a bold assumption, and then discuss this issue based on that assumption.
The assumption is that you are being honest, and actually know and have dealt with people who have drug problems, and were instructed in eternal security.
In the case of a person getting into drugs, and assuming they are saved, they have one of two possible issues:
1. They have misunderstood OSAS, and have continued living in sin because of their immaturity and lack of understanding. They mistook the grace of God for a licence to sin, and obviously have no Spirit within them, guiding them to maturity.
2. They have been instructed in eternal security from an immature carnal minister, who twisted the doctrine to please his own flesh, and to pervert those around him to act in the same fashion. If they had the Spirit, they would not have accepted such perverse teaching.
The point is this: every doctrine has groups of people who twist them into whatever they want. People do the same with the Word of God.
Just because a doctrine has been twisted by an evil man doesn't mean it is false.
Satan twisted several Scriptures while tempting Christ, but that doesn't make those verses false.
This is not foundational teaching, but a part of growth and maturity. If you spend enough time in 1,2 Corinthians, or in the Gospels, or in Romans, you will eventually see how secure salvation is, and will understand.
This doctrine is not needed for salvation, but once you understand it, you will understand a bit more of what the peace of God really is, and why it surpasses all understanding.
If you are not growing past working your salvation out on your knees, then how can you grow up as a mature Beleiver?
If you are feeling the need to be saved again on a regular basis, then you are still in babe state.