Just because 1 drug is more potent doesnt make it better or worse. In example 10mg of drug a could equal 100 mg of drug b. So doctors just prescribe according to the ratio.
Not fully true. An amount of codeine equivalent to demerol would rack you with side effects. A better general rule is that more potent version of drugs are more efficacious than their less potent counterparts as side are usually diminished for a given therapeutic threshold.