As per http://www.aarcmcc.org/docs/Electric...0July%2008.pdf

3.1.7
A receiver battery pack must only supply power to radio receivers for its operation......

Kind of makes sense seeing that its a "receiver" pack and when its plugged directly to a speedie you dont really know what the speedie is doing with the power. Although im sure the tekin is faster with a receiver pack, although that could be because the speedie is browning out without the pack (like a novak smart boost does with the tekin).

ROAR rules are not as specific:
Receiver and servo power for electric vehicles may come from an electronic speed controller equipped with Battery Elimination Circuitry (BEC) or from a separate battery pack used for this purpose only. (See rule 1.5.19).

However, to me the same stands, as my understanding of the concept of a BEC is take the main battery power and (usually) drop it as required to provide power to the receiver. It certainly doesnt say that a receiver pack can be used to provide power to an esc.

Ooops thought this was the australian thread, but i still believe my comments are correct :P