One of the difficulties that comes when one concludes that we have a "sin nature" that is not describing the corruption of our flesh is that we have to then read passages of the Bible as if they don't mean the exact thing they are saying. How much of a habit do you want to make of doing that?The fundamental sin nature, that propensity to live in corrupt desires of the flesh, is discussed here:
"5Those who live according to the flesh set their minds on the things of the flesh" Romans 8:5
I agree that the NIV's phrase 'sin nature' and the way Christians have used it is misleading. The problem of a 'sin nature' is really a mind and soul thing rather than a flesh thing. When one is born again that mindset gets replaced with the mindset of the Spirit, and all that remains is the flesh and it's desires.
Romans 7:5 KJV
5) For when we were in the flesh, the motions of sins, which were by the law, did work in our members to bring forth fruit unto death.
"Members" here is literally "body parts". When we were in the flesh refers to when we lived a life derived from living in our bodies. Now we are in the Spirit, and we have life derived from the Holy Spirit. So that should the body die, we continue in life, His life in us.
Romans 7:14-25 KJV
14) For we know that the law is spiritual: but I am carnal, sold under sin.
15) For that which I do I allow not: for what I would, that do I not; but what I hate, that do I.
16) If then I do that which I would not, I consent unto the law that it is good.
17) Now then it is no more I that do it, but sin that dwelleth in me.
18) For I know that in me (that is, in my flesh,) dwelleth no good thing: for to will is present with me; but how to perform that which is good I find not.
19) For the good that I would I do not: but the evil which I would not, that I do.
20) Now if I do that I would not, it is no more I that do it, but sin that dwelleth in me.
21) I find then a law, that, when I would do good, evil is present with me.
22) For I delight in the law of God after the inward man:
23) But I see another law in my members, warring against the law of my mind, and bringing me into captivity to the law of sin which is in my members.
24) O wretched man that I am! who shall deliver me from the body of this death?
25) I thank God through Jesus Christ our Lord. So then with the mind I myself serve the law of God; but with the flesh the law of sin.
This entire passage is given to teach us the distinction of when we were in flesh, and now that we are spirit. For I know that in me (that is, in my flesh) lives no good thing.
He speaks in specific terms of the inner man, and the flesh, consisting of our members, the body of this death. All these terms are of our physical being.
Ephesians 2:3 KJV
3) Among whom also we all had our conversation in times past in the lusts of our flesh, fulfilling the desires of the flesh and of the mind; and were by nature the children of wrath, even as others.
The desires of the flesh, everything from being hungry or thirsty, to lusting for sex or money or power, whatever the desires of the flesh are.
There is much in Scripture like this.
Why is it important? It's what the Bible says, and we should accept what the Bible says. It demystifies sin and temptation. There is not some "unseen power" looming over us, a "sin nature" that somehow has power to reach into your life and mess you around. Our bodies are corrupted, damaged. One day that damage will be undone, and we will be free of it's lusts. But in the meantime, that's all it is, a broken body that is perverted (wrongly bent) towards evil, but we have the power to control it, because we have become alive in Christ.
Much love!