Yes, if you are a regular club runner who partcipates in a County League or club championship league. But it can normally be worked around.
The worst bit is trying to train for a marathon and still maintain a good race on tired legs.
I suppose the answer is to have a few target races that you want to peak for and do the others as fast training runs.
In fact I personally believe that lots of racing throughout the year can help you get into great shape!
