Lets be scientific about it. Lets propose a hypothesis. Lets take a sample of games at intermediate level, say 100.
How many games do players lose statistically as a direct consequence of a serious mistake from the opening?? How many do they lose statistically in the middle game? How many do they lose in the endgame?
Perhaps that will answer how important openings are in comparison to other areas?


If you blunder in the middlegame the opening probably won't matter. You can end up in a situation in which your opening knowledge doesn't make up for your mistakes in the middlegame and your time will end up being used for the wrong things. This is one reason why openings become more important as your rating gets higher.
You shouldn't completely ignore them but there may be more efficient ways for you to improve.