As long as the commentators keep the comparison to just the TdF stage wins, I don't really have a problem with them recognizing that Cavendish could surpass Merckx. From what I've heard from the commentators, they have not tried to imply that Cavendish could accomplish anything besides that record during this TdF. I live on the west coast and so by the time I turn on the tour in the mornings I'm only catching the last 60-90 minutes of the stage and so I haven't heard 100% of the conversations. However, given how long Merckx's record has stood, I think it is worth dedicating some attention to the topic. Have the commentator gone overboard? I think that is probably a matter of opinion but I don't think that they have tried to imply that Cavendish is a better cyclist than Merckx, only that he could break the record for stage wins.
The other point of view I try to take on topics such as this is what if the commentators didn't pay any attention to Cavendish's potential to break a record held by a cyclist as great as Merckx? If they completely overlooked it, wouldn't that make them incompetent commentators? What is too much recognition? What is too little recognition? I know that if I lived in England that I would appreciate the recognition that the commentators were giving my fellow countryman.