Pros and cons both ways (like most things) ... I've tried it both ways, explain it both ways to my students, and let them decide which works best for them.
Basically, the reasoning behind using your drysuit for buoyancy control boils down to simplifying management ... it's easier to inflate and vent one source than two. There is some merit to this, although if you leave the dump valve on your drysuit open (or mostly open on adjustable valves), then dumping air from a drysuit pretty much manages itself ... when you lift the BCD inflator hose to dump air, the left shoulder (or wrist, for those with wrist dumps) becomes the highest point on your drysuit, and the place where the air bubble in your suit wants to go. It will come out without any conscious effort on your part.
The reasoning behind using your BCD for buoyancy control boils down to stability ... as you move, any trapped air is going to travel to the highest point it can get to, and a BCD bladder offers much less ability to move around than a drysuit. Keep in mind that as that trapped air travels, it affects your trim, and you have to compensate with body movements to maintain good trim.
Now, a properly weighted diver is weighted for the buoyancy of their cylinder at the end of a dive ... which means at the beginning of the dive you will be several pounds overweighted. You have to compensate for that by adding air to your BCD or drysuit ... whichever you're using for buoyancy control. So it's at the beginning of the dive where which choice you make will matter the most. With a smaller cylinder, such as an AL80, it amounts to about 6 lbs of extra weight that you're compensating for, and that's a reasonably manageable bubble. With larger cylinders, the difference can be more than 10 lbs. With doubles, it can be considerably more than that. The larger the bubble, the less easy it is to manage it using your drysuit ... especially if you have to break trim and get vertical for any reason, at which point all that air comes up around your shoulders ... and possibly starts to bubble out your neck seal.
For the recreational diver, it really boils down to choice ... use whichever method is more intuitive for you. For the more aggressive diver using larger cylinders ... or for the recreational diver who's grossly overweighted to begin with ... using the BCD is advantageous.
... Bob (Grateful Diver)