I've got a matched sub with my monitors... the woofers on the monitors are only 6.5" and didn't really give me the proper bass response that I was looking for, so hence the sub.
If it is tuned properly to the room and you are used to it, it can definitely help.
Most people say they shouldn't be used all the time, but personally I'd rather use mine than be completely unaware of what's going on in the low end.

Soon I am going to get the JBL's though and use them without a sub as the woofer size should put out enough bass.