Alright... So you dive dry and without a BCD... Let me ask you this...
Let's say that you're diving twins one day... You know that with your drysuit and with twins (whatever size they are) on a backplate only, you need 15 lbs to be perfectly neutral at the end of the dive. So you wear your drysuit, a backplate, twin tanks, and a weight belt with 15 lbs on it. This seems to set you up nicely for trim, too.
So you get into the water at the beginning of the dive... And you decend to 100 fsw onto a wreck that you were looking forward to. Obviously, this is the point in which you are the heaviest during this dive, because your tanks are full. Thus, you need to counteract the heavy tanks to remain neutral. How much, exactly, would that be? Well... It depends on the size of the tanks, but even if they're wimpy AL80's, you'd still be 12 lbs heavy, and thus would have to put 12 pounds in your BC to be neutral. Of course, you don't HAVE a BC, so you put 12 pounds of lift in your drysuit.
So there you are, nice and horizontal, with 12 pounds of air in your drysuit... What happens when you go and look down into a porthole or open hatch in the wreck? Every bit of air that you have in that suit rushes to your feet! Thus, you are hanging there in the water column, with all of the air in your dry suit around your ankles... And unable to vent it, too, so that you can right yourself...
Why dive with a BCD even when you have a drysuit? Well... Because it works best for trim...
Drysuits are a nice emergency source of lift... But you don't want to use it as a BC... It won't do much for your trim...
Imagine the above scenario with tanks larger than AL80's...