The thinking is in error ... drysuits are designed to vent more slowly than BCDs, so once you begin an uncontrolled ascent, it will be harder to get under control by venting the suit rather than the BCD. In fact, under certain conditions you'll reach a point where the air in your suit is expanding faster than you can possibly vent it ... in which case you aren't just going to the surface, you'll be accelerating the whole way there. Besides that, think for a moment about where the drysuit dump valve is located, and why they'd put it where they did ... in an uncontrolled ascent while dumping air from your BCD, you'll also be dumping it from your suit without any additional effort (assuming you've got your dump valve adjusted properly). By using the suit only for buoyancy control, you don't have that option.
The theory behind recommending suit for buoyancy control is for ease of air management. It's a nice theory, but in practice it tends to present more downsides than upsides. Sure, it's easier to learn that way ... but there's a reason why most experienced drysuit divers tend to only put sufficient gas in the suit for warmth and range of motion ...
... Bob (Grateful Diver)