I can help.
Without taking sides on the carbon / no carbon issue, tank odor or discoloration of the water can signal some kind of problem within the tank that requires the attention of the fishkeeper. The argument is that by removing the odors and discolorations, the root cause of the problem might go undetected.
Just as an example, a strong 'fishy' odor can be a sign of excessive dissolved organic compounds which when they decompose can pollute the tank with ammonia, causing a dangerous spike.
The argument is that by removing these odors, the underlying condition, like too many dissolved organic compounds, might go undetected -- continuing to pollute the tank for a longer period of time than it would have had the odor alerted the fishkeeper to the issue.
This is just one possible example out of many, but it is this type of thing that people are pointing to when they say that using carbon can mask a problem going on in the tank.
Again, I think there are pros and cons regarding regular carbon filtration in aquariums -- but that's the essence of the argument as I understand it.
I hope this has helped.
Best!