OK. All of these arguments are well and good, but let's look a little bit at the "perfect, infallible" Bible (note that I do not say Word of God).
I have a friend who recently had a terrible experience: his penis was cut off as part of an operation due to complications from an anti-depressant drug. The Bible teaches that:
"If a man's testicles are crushed or his penis is cut off, he may not be included in the Assembly of the Lord." Deut. 23:1 (NLT). Is my friend excluded from salvation? I think not.
The "perfect and infallible" Bible includes such gems as:
"If two Israelite men are fighting and the wife of one tries to rescue her husbanc by grabbing the testicles of the other man, her hand must be cut off without pity." Deut. 25:11.
Mark 16:9-20. (Longer Ending) Of course, neither the Pope nor any leader of the envangelical protestant movement would rationally test their faith by drinking poison. We all know what would happen.
But the Bible is replete with such stuff. Does it contain wisdom for better living? Undoubtably. Is it infallible having been written by man? There is much to wonder about.
There are numerous, and in some cases very specific reasons, why the Law was given such as Deut 23:1. However, none of these Laws offered salvation. They only pointed out that it is impossible to keep many of these Laws. (e.g. How would you verify such a situation in your church?)
As far as "drinking poisons", I can assure you that if I drank hydrochloric acid or even worst my own cooking, I will not live to see another day. This part of Mark is in dispute. That doesn’t mean it’s not inspired although some text do not include it. It means that we don’t know what it means. There are a few of these odd verses sprinkled here and there. People will take one tiny verse, sometimes a segment of a verse, and build a whole cult around it. All of a sudden you have people grabbing cobras and having a bewildered, puzzled look on their face right before they hit the road for glory. Meanwhile the cobra caulks up another victim right before everyone gets mad at it. Stupid is as stupid does. It's not scripture but it's none the less true.
Protestants believe scripture interprets scripture; that is, if you find an odd passage of scripture that seems out of place look for another piece of scripture that clarifies it. Since there are no other scriptures similar to Mark 16:17-20 it best to simply leave the verses alone. I will say some commentaries interpret this to refer directly to the apostles only, which seems to be a valid interpretation. Personally I shrug my shoulders and move on.
There are many people, including Christians, who do not believe the scriptures are the infallible word of God. Just take a look at this post. To me this is ludicrous. I can understand an atheist or an agnostic being skeptical of the scriptures but not a Christian. Either you believe it by faith that it is the power of God to change lives or it is nothing more than a book of fables and stories. There is no middle ground.
There are many, including myself, who will attest to it fundamentally changing our lives. But you must acknowledge your sin before God and accept His authority through His word by faith. Those are the rules. We walk by faith, not by sight.