משנכנס אב ממעטים בשמחה
During the 3 weeks and especially during the 9 days we observe many ניהוגי אבילות such as not eating meat, not listening to music, not bathing, etc. The point of these is to make us sad and help us focus on the aveilus of the Churban.
The ניהוגי אבילות are not an end to themselves but rather a means to an end. They are supposed to bring us to a mood of aveilus. Therefore, for people to rely on kulos or work around these halachos completely defeats the purpose. Here are 2 examples:
1. Eating gourmet milchig meals with ice cream (because we are milchig) for dessert. This fulfills the ניהוג אבילות of not eating meat but defeats the purpose, you will not feel sad or missing anything after a gourmet meal. Someone who does this has completely missed the point of the 9 days.
2. Al Capella music. I am not going to get involved in the Halachic issue of whether it is permitted or not. Even if it is strictly speaking permitted again, it is against the spirit of the period and defeats the purpose. Presumably, a person listens to this music because he enjoys it.
It is hard for those of us who grew up in the West to relate, but during the 9 days a person is supposed to deprive himself and suffer a bit. This period is supposed to bring us to a state of mourning for the destruction of the Beis Hamikdash and the Galus Hashechina which is the root cause of all our problems.