Actually, you do use the bong that Dan Chi Sau teaches... every time you do two-handed Chi Sau.
All Wing Chun drills should be dependent upon the attributes and skills that are being developed. IMO, it is in our best interest to keep in mind which Wing Chun concepts and principles the drills are teaching us as they relate to the system as a whole. For example, Dan Chi Sau develops Yi Bong, and the Lap Sau Drill develops Paau Bong (among other things). These are two completely different uses of bong (with different energy and projection) that need to be properly established separately. If you remove Yi Bong from Dan Chi Sau, you are changing the drill’s purpose of development. Likewise, leaving out bong no longer allows Dan Chi Sau to be utilized as an effective training tool for two-handed Chi Sau... causing a ripple effect.
Dan Chi Sau is typically trained with a midline counter punch from the Jam Sau for several key reasons:
To develop shifting/rolling bong action against pressure, to train proper pinning elbow position for correct controlling and hitting, and most importantly as a precursor to two-handed Chi Sau (again, part of which is reinforcing Yi Bong development).
That said, if your training partner fires off a solid midline counter punch from a proper Jam Sau in Dan Chi Sau, a Biu Sau response to a midline attack would most likely get you hit. At the very least, you’re setting yourself up to trade hits. And that's not a good training habit.
Respectfully, have you thought about introducing your Biu Sau response in Dan Chi Sau as an additional action (and not omitting bong altogether)? As an alternative, you could keep the midline attack from Jum Sau so the drill still develops Yi Bong, but also add a separate high attack to allow for your Biu Sau counter. That way you can use Biu Sau in Dan Chi Sau and it will not come at the expense of leaving out other key attributes or skill development, such as bong and Chi Sau.