IMO, and others, there's a couple of problems with long term use of carbon.
It might work too good and remove some desirable trace elements and nutrients.
Also, eventually, it needs to be replaced and or cleaned. The charcoal has such a high surface area that a lot of the beneficial bacteria live on it. So when the carbon is replaced a lot of your bacteria is lost.
There are ways to avoid these issues. But I prefer to run the carbon once a week or after I've done some fraggging or added a new coral.