The reason SBC churches require baptism by immersion for membership is simple: The only way anyone in the Bible was ever baptized was by immersion....
Jesus himself was baptized by immersion... There's no record of a single non-immersion baptism in the Bible... the word "baptize" itself means "to immerse"...
Personally, I find it to be no big deal. I think as long as it was a CONSENTUAL, confessing and willing baptism (i.e. not infant baptism, old enough to make the choice), it should be valid... but this is a minor point, as there is nothing considered wrong with being baptized again, but you do not have to be....
My church, a Bible-based church with very similar values to the SBC (
http://www.fotw.org) also requires baptism by immersion for any member who has not received this at some point in their life.... We have an awesome baptism pool that is modeled after one on the River Jordan, near where Jesus was baptized.... Baptism ceremonies are held frequently and it's an awesome experience.
Hopefully that helps some. Oh, BTW, I was raised Southern Baptist. Most of the Baptist churches in the U.S. are affiliated with the Southern Baptist Convention, if not multiple affiliations....
-Michael