Justifications for God's ordinances and rulings are not always what they seem to be, and many people were lead astray thinking about obvious benefits that they spotted instantaneously in something God ordained, that if it turns out wrong they sort of stop there confused and some even start doubting.
Ordained slaughter was never claimed to be a painless experience. The justification made though is that it is cleaner, which was proven by science beyond a doubt: slaughtered animals pass out from the cutoff blood while their hearts continue to pump blood OUT of the body of the animal through the slit jugular, thereby reducing greatly the amount of blood that resides within the body. All analysis have found that dead blood is very unhealthy, has toxins, and should not be ingested.
Is slaughter painless for the animals? Don't know and never was said Islamically, but these are cattle that are part of God's bounty for us, so our consumption of them is normal and takes precedence with us as muslims.
Like Gossamer said though, I can hardly imagine an argument to be made against slaughter when the alternative they take is blood-curdling electrocution, proven to be so maniacally painful that some chicken bodies were found to have convulsed so hard they dislocated their own joints.