You are correct. However, there are instances where on a parlay card when you may be getting significant value on certain lines and you may want to go that avenue. You will have to do the math on it and see if it is worth it or not to bet the parlay card at the reduced odds with better lines or to bet it off the board.
PS- PLEASE BE ADVISED TO READ ON THE RULES ON YOUR PARTICULAR PARLAY CARD. IN OTHER WORDS, READ THE SMALL PRINT!