Hmmm...I would agree if an instrumental melody was played in minor, while a singer sung in a major scale, and all this over a major key progression. Even writting it out sounds bad. Actually using a minor after a major melody is not wrong. Since music is made of two parts, harmony and melody, they can either play with each other or against each other. Music doesn't have to be strictly linear, changing the texture of an idea by doing the above increases the signifigance of it. Sort of like introducing a character in a story but putting them in different situations to learn more about the actual character. Music that becomes more tense, or vice versa, becomes more interesting. Yes care must be taken, like I said it's more advanced and any beginner should be first comfortable with playing with the harmony before trying to play against it.