Whoa, you seem to be saying Baptists (who get their name from the importance of believer baptism) and Pentacostals don't care enough about one of God's commanded rites. They don't think baptism is any less important than it is for those who have been saved. It is all about timing: infant baptisms are useless and meaningless because they have no effect on a baby's chances of being saved, so their policy is to wait for people to make the conscious decision themselves to follow God and receive the Holy Spirit.
I don't want to get into a debate on infant baptism because there's so much that goes into, and I'd like to stay on topic. Basically, in the Arminian Baptist or Pentecostal system, of course infant baptism doesn't make sense. The whole system operates around that thought. Other systems that are more theologically conservative strongly disagree. The sum of it is: is baptism something we do for God? Or is it something God does for us? At least Lutherans will say that baptism is something God does for us, therefore we should not hinder anyone God's grace. So hopefully you can get a sense of the difference.
I'm not saying that baptism is unimportant in the Baptist or Pentecostal system, for I know it is. I was raised Pentecostal. But compared to orthodox branches, they are less important - both theologically and practically, and I can understand if that sounds strange. Let me demonstrate:
Baptists and Pentecostals generally view (water) Baptism as a symbol of new life in Christ; an outward declaration of faith; an act of obedience to Christ. Orthodox bodies believe that it doesn't only symbolise, but that it truly is God's means of grace. Therefore they will say baptism is (ordinarily but not absolutely) necessary for salvation. That's why orthodox branches will say that if we're baptised, there is no need for re-baptism if we were to fall out of faith and come back. For once we're baptised into the name of the Father, the Son and the Holy Spirit, it stands eternally on account of God's own promise through baptism. This is apprehended through faith.
Baptists and Pentecostals view the Eucharist more or less as a symbol, and it doesn't confer any actual grace; it's memorialism; eating bread and drinking some wine, reflecting on Chris's life, death and resurrection. Orthodox bodies believe that it's not only a symbol and occasion for reflection, but that it truly is God's means of grace. It's truly the body and blood of Christ given for us; a gift from God for our comfort, apprehended through faith in the person and works of Jesus Christ.
Bodies after the Zwinglian system view Baptism and the Eucharist as symbols, therefore not very central in their theological thought. A great example of this is some Baptists' insistence on full immersion, where the outward practice is more rigorously defended than the actual essence of what baptism is. Orthodox bodies view Baptism and the Eucharist as true means of grace, whereby the Holy Spirit operates and confers faith. They are not mere symbols, but synonymous with the Gospel; God's grace for us through Jesus Christ.
Now, I can appreciate that much of what I said here may seem confusing (or not!), but rather than debating me, I would encourage you to read about the Reformation and what they confessed. It's a real gold mine of theological thought that has shaped the Church as we see it in our day. We can get a better sense of what is taught in the Roman Catholic (and to an extent Eastern Orthodox), Lutheran, Anglican and Reformed and their consequent bodies such as Baptist and Pentecostal.