Even the ones who understand it's not needed for Salvation, have this issue where they add other requirements besides profession of faith for it. Basically in response to others who Baptize everyone and everything without consent, many Baptists want to put all kinds of restrictions on Baptism.
Most even Independent Baptists require you to become a member of their Church (I don't think Churches should even have official membership lists), which in turn often requires an agreement (that they usually don't really try to enforce) to live by a bunch of rules.
Even if not requiring membership they feel that they have to ask all kinds of questions, to make sure you're ready for a Christian life.
Before things like Infant Baptisism became an issue these problems were popping up, The Didachae around 100 AD was saying you should fast for a day before Baptisim.
Let's look at Acts 8: starting in verse 36
And as they went on their way, they came unto a certain water: and the eunuch said, See, here is water; what doth hinder me to be baptized? And Philip said, If thou believest with all thine heart, thou mayest. And he answered and said, I believe that Jesus Christ is the Son of God. And he commanded the chariot to stand still: and they went down both into the water, both Philip and the eunuch; and he baptized him. And when they were come up out of the water, the Spirit of the Lord caught away Philip, that the eunuch saw him no more: and he went on his way rejoicing.No membership, no joining any earthly group. No making sure he got various Doctrines right. No asking him what he was willing to give up for Jesus. No asking him how he intends to live his life. He just asked him if he believed, and he believed, and that was it.
People also have this notion that Baptism is a public declaration of Faith in Jesus. This wasn't Public, they were alone, no one else saw this.
John The Baptist alone Baptized people Publicly. Paul defined John's Baptisim as distinct from the water baptism the Church practiced after Pentecost.
Acts 13:24 When John had first preached before his coming the baptism of repentance to all the people of Israel. And as John fulfilled his course, he said, Whom think ye that I am? I am not he. But, behold, there cometh one after me, whose shoes of his feet I am not worthy to loose.
Acts 18:25 This man was instructed in the way of the Lord; and being fervent in the spirit, he spake and taught diligently the things of the Lord, knowing only the baptism of John.
Acts 19:3 And he said unto them, Unto what then were ye baptized? And they said, Unto John's baptism. Then said Paul, John verily baptized with the baptism of repentance, saying unto the people, that they should believe on him which should come after him, that is, on Christ Jesus.When they heard this, they were baptized in the name of the Lord Jesus.
Lots of Baptists don't understand this, I'm kind of disturbed how often Baptists act like John not Jesus and the Disciples was the founder of their religion. John is defined Biblically as the end of The Old Testament, not a beginning.
They think Apollos only needed to be Baptiszed again because it doesn't really count if you weren't Saved when you were Baptized before. I think it is good to Rebaptize converts from other forms of Christianity. But that isn't the point here, the emphasis was on it being of John rather then Jesus.
When we receive the Holy Spirit we are Baptized spiritually, not with Water. The Water Baptism The Church does is mainly an outward symbol of that Baptism, as well as being "Buried with him in baptism, wherein also ye are risen with him through the faith of the operation of God, who hath raised him from the dead." Colossians 2:12.
A lot of Baptists seem really afraid of accidentally Baptizing someone who's not really Saved. But why care? All you did was waste some water.
As far as the error of thinking Baptism is necessary for Salvation. There are really only two verses that doctrine is built on.
Mark 16:16 "He that believeth and is baptized shall be saved; but he that believeth not shall be damned."
The condition of those that Believe but aren't Baptized isn't addressed either way. Clearly this verse alone doesn't settle the issue. Some might argue that this means the salvation isn't secure until Baptism. I think the reason Baptism is excluded from the second statement is to make clear that Baptism without belief doesn't Save.
Acts 2:38 "Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost."
First of all, the Context of this being Pentecost and addressing the Gifts of the Holy Spirit tells me this isn't Water Baptism but the Baptism of The Holy Spirit.
Secondly, nothing said here makes it needed for Salvation, on Salvation he just said "all who call upon the name of The LORD".
I also feel many people added to The Church at this time were already Saved, I think a lot of the early mass conversations were merely bringing into The Church those already Saved. Because I'm not and have never been the kind of Dispensationalist who thinks Salvation used to work differently. Paul proved his doctrine of Salvation from The Old Testament, where Baptism never existed. I disagree with the claim of some Hebrew Roots people who think it came from the Mikvah custom.