Ive read through page 7 of this thread (and intend to finish it) and I havent yet seen anyone bring up the soundest reason of all for baptism:
When we accept Jesus, we take Him as our Savior and Lord. If we mean anything by the word Lord, we mean that we will do, to the best of our ability, what He says to do.
Starting with John the Baptist, Scripture has passage after passage, most of which have already been quoted, saying Repent and be baptized, He that believes and is baptized, and so on.
Now, nobody whatsoever is saying that getting dunked under water is by itself going to save you if that were the case, a bully-type kid that used to swim at the same city pool as me would be one of the worlds premier soul-savers! (insert tongue-in-cheek-smiley here)
Rather, what happens is that you turn to God in grateful acknowledgement of His grace, and accept Jesus as Savior and Lord, and then do what He says. And one of the things He says is to be baptized. The Great Commission sends us as disciples to all the world, proclaiming the Good News and baptizing in the name of the Father, the Son, and the Holy Spirit.
Is it 100% necessary for salvation? Nothing is we are saved by the grace of God and the Atonement of Christ, not by anything we ourselves do. But were called to believe, to repent, and to be baptized. The hypothetical guy who finds God while dying in the middle of the Sahara is saved by Gods grace, nothing else. The guy on his deathbed, the same thing.
Theres a passage in Romans that says He who believes in his heart, and confesses with his lips, that Jesus is Lord shall be saved, or words to that effect. From this we do not conclude that no mute person can be saved. But we do conclude that a person able to talk who believes had better be confessing Him with his/her lips.
The same thing holds for baptism. If one comes to know the Lord, and it is physically possible for him/her to be baptized, he/she is obliged to obey Gods command and be baptized. That somebody is dying in the Sahara or a car wreck whom God will save because he/she came to know God and had no opportunity for baptism is no excuse for us.
I might note that the Roman Catholics had a rather sound doctrine on this: while all Christians should be baptized, a believing person who is unable to be baptized, according to them, receives the baptism of desire because he/she wills to follow Christ, but is unable to carry out this particular command, He takes the desire for the deed. Likewise a martyr for the faith not yet baptized is considered to have received the baptism of blood his blood shed for Christ baptizes him as he dies. None of this is Scriptural so far as I know, but it rings true to the grace of the God who knows our weakness and loves us nonetheless.