Some confusion may be due to my lack of knowledge on the subject. I (perhaps incorrectly) used the "DCC" term to cover all digital systems and "DC" to pertain to analog systems. I'll try to rephrase:
My understanding is that with analog (DC?) three rail, the outer rails are always the same polarity and the inner rail is always opposite in polarity. This functionality facilitated the "toy" segment (whistle, uncoupling, etc.).
Since digital systems function with two rail layouts, does the center rail still offer any functional or feature advantages over two rail?
Hope this makes sense.
In most 3 rail, even command (digital systems), the center rail is a different polarity than the outside rails (Outside rails are tied together electrically in most track systems.). The center rail allows for simple powering of return loops and other complex track layouts. I don't know about the "toy" segment you speak of. By keeping 3 rail by going to command (Like DCC.) you still keep the ability for simple powering. Instead of wiring a left and right rail, you are wiring center and outer rails. The command systems don't care if you have 2 rails or 100 rails, as long as you have a total of 2 polarities.