You are rewarded more during the main story quest which is only doable once per character. If any merging happens, classes and achievements should not be merged. You should choose one character to be the final character, and all levels, achievements, minions, event records etc on other characters are ignored and lost.

Which would defeat the purpose of a character merge.

Having different characters allows you to progress faster in some ways (such as double lockouts, double weekly caps). The fruits of that advantage needs to go away along with the character merge.