If you have e.g. 40q on attack and 20q on defense, the game won't let you add more skill points into the attacking qualities until you raise the player's defensive ones.
On topic - the only thing that I've observed in my players is that those of them who have low defensive skills tend to get more yellow cards, especially for the MCs - I have one that already got suspended twice in the league (+ he has another yellow now), once in the cup and will miss the CL final due to another suspension.