Hi, just to provide some additional clarifications.
As a note, ANT master channels do typically take a brief receive window to find an open timeslot before broadcasting, so they do have some measure of avoiding each other even as they start transmitting.
In a shared channel environment, slaves can only begin transmitting to the single master when the master is directly addressing it, which if you wanted to poll each slave at a 1Hz rate and you have 300 slaves, the master would need to transmit at a minimum to 300Hz, polling each sensor at each period.
The combined message rate specification is different from the co-existence rating. A CC2571 can communicate at generally a combined message rate of 200-250Hz across all channels on the device combined. So even in continuous scanning mode, you would see generally around ~200 Hz of broadcast/acknowledged messages. The 300Hz density rating dictates how many CC2571's can broadcast in the same RF channel without interfering with each other.
The advantage of shared channels is that the master is allowed to remain as a synchronized channel saving power, but that the master must decide how often and for how long it wishes to communicate with each node.
The advantage of continuous scanning mode is that there is no latency involved at the central receivers end, and the external nodes can decide when they wish to transmit. An ANT continuous scanning node can remain fully bi-directional so the master nodes can still use Acknowledged and Burst data types, so your external nodes could just wake up, Ack/Burst until it's successful and then turn off again.
One interesting topology I have seen used is a product where two ANT devices are used at the main receiver end, one sits in continuous scanning mode to receive constant data from sensors, but uses the other device to pull history data over ANT-FS from each sensor simultaneously on a different RF channel.
The other topology was dynamic switching between continuous scanning and ANT-FS on a single ANT device for pulling activity data which was stored over the course of a couple weeks from sensors passing by a receiver. The receiver would use continuous scanning to find all the devices which had data to be pulled, switch to ANT-FS to pull the data and tell the device to stop broadcasting that it had new data for awhile, scan, pull, etc.
Glad to have helped! :)