Over the course of my diving career, I did a full 180 regarding my stance on this. I started diving my drysuit purely in a recreational diving arena. It was very comfortable and easy to use just the suit for buoyancy, but I found later that it probably isn't the best way to go, and that I had likely developed some very tough to break habits by diving that way.
When I started diving doubles and sidemount, still all at recreational depths, it was still pretty easy to maintain buoyancy with just the suit, but I started noting that it was requiring quite a bit of air on every dive just to run the suit. This was on normal open water dives, where depth changes, especially need for repetitive ups and downs, were rather limited. Where this became undeniable, however, was when I used a drysuit inflation bottle with a reg setup that didn't have a dedicated inflation hose for the drysuit - I quickly found the bottle empty. As I moved into cave diving, that is where I really started noticing a difference.
Cave diving involves a lot more changes in depth that aren't exactly optional. If the cave goes up and down, so do you. I noticed that in areas 60 foot or less, when there was a swing in depth of 15 feet or more, it started getting very difficult to keep good control of my buoyancy, and I was constantly adding and dumping gas from my suit. In trying to keep my position steady in relation to the cave ceiling and floor, it became painfully apparent that I needed to work hard to stay where I wanted to be. I thought this was all just part of the process of getting used to the task loading of cave diving, though, and didn't consider that my chosen management of my drysuit was part of the difficulty.
Where I finally had my lightbulb turning on moment, though, was when I started diving a rebreather. During my initial classwork, there were a lot of dives in the 60-100 foot range, and the instructor intentionally had me changing depth -- a lot. With the rebreather on, I needed to add a TON of gas to the suit at depth, and even a little to the BC to keep things level. When I would ascend, dealing with the suit, the BC, and the rebreather loop started getting a little overwhelming -- particularly since you can't use depth of breathing to adjust your buoyancy on a rebreather. I also noticed that I continued to drain my drysuit inflation bottle every dive, as this was now a standard thing for me to use with the way I plumbed my rebreather. Around that time, my instructor told me "hey, it looks like there's a lot of gas in your suit." But...but...yeah, he was right.
From that point forward, I started adding just enough gas to keep the suit squeeze comfortable, and started adding gas to my BC to manage buoyancy changes. This made an immediate and obvious difference in regards to how easily I could manage depth changes. It also markedly changed the amount of gas I was using each dive. The suit is a LOT bigger than the BC, after all. All in all, I would be minimizing things to say it was a night and day difference how much easier and better things seemed to work using the BC for primary buoyancy control.
Where I KNEW it was better to always run the BC for buoyancy control, however, was when I did my next open circuit cave dive, and I tried the BC for control strategy. I had a significant improvement in my penetration distance before I hit 1/3rd of my tanks. I suppose this is because I was using less gas to keep my suit inflated, and even more importantly, it was easier to swim since I wasn't pushing such a large profile through the water. A suit that was not as full all the time was significantly easier to swim, and I remember noticing that it felt like I was "lighter" in the water. That led to me being actually lighter - as I was able to drop the trim weight I had been carrying on my subsequent dives, since I had less air in my suit to offset with lead. Then I started wondering why I ever did it with just the suit in the first place.
I suppose the answer to that is easy - running buoyancy with just the suit was simple and effective at recreational depths and wearing a single tank, and I never knew the shortcomings it would have later on in my diving career. In fact, I bet if I never did tech diving, I would still be using my suit only for buoyancy control, I would feel comfortable that I had it all "figured out," and I'd likely teach my students the same way. If this is the way you intend to dive, and you know that you are never going to do any tech diving, then by all means - feel free to stick with the suit only as your buoyancy control at depth. If you are just starting out with diving a drysuit and you feel like the task loading is too high, then for now, feel free to use the suit until you are getting the hang of managing an extra air space. But if you think that SOMEDAY you might want to dive in a tech config, or you are feeling very comfortable managing your drysuit, maybe it would be worth trying a little less air in the suit, and a little more air in the BC - just to see how it treats you. Who knows, you may even like it! And maybe it will be paving the way for a less painful learning curve in the future if you advance in your diving.
Even if you think that you will never do tech diving, take a lesson from me - I told myself as little as 2 years ago that I would NEVER be interested in cave diving. And now, I can't get enough. The thing I have learned time and time again with diving is that I can never say "never" in regards to where I think my diving will take me. The more I experience, and the more I learn, the more I realize that the trajectory of my diving career is always subject to change. Who knows where my diving will take me tomorrow?