I think it depends on what type of cruise you are on and whether or not you go to the events anyway. If you're on a small ship and/or a itinerary with cold weather, then a good CD plus a good Activities Manager is imperative to make sure you have things to do when you can't go to the pool or the other amenities (if the ship even has them). The Activities Manager is equally as important since they manage the trivias and all of those events and sometimes a decent AM can make up for a below average CD.
On a warm weather cruise on a big ship, probably less so but some of us still enjoy going to the events. A CD doesn't make or break a cruise for me but they can affect certain events such as the Love and Marriage or theme nights which I like. And I also enjoy a decent morning show on the TV, the likes of Abe Hughes & Leigh Xuereb (neither are Royal CDs any more) were great with that but my most recent cruise on Odyssey, the morning show was just so basic and lasted only a few minutes.
It's also personal preference as to what style of CD you like. I've had 12 CDs in total, 9 I've liked and there were 3 who were not my cup of tea. It doesn't make or break a cruise but it's better when you get someone with a bit of energy and humour and seems somewhat engaged with guests (wouldn't judge them on how visible they are though since the mega ships require a lot of behind the scenes work).