Just to clarify, the old HASTE system that was used to provide notification of an incoming medic unit to a hospital, was a one-way voice message from the dispatcher (as Steve162 pointed out above), and not the medic unit. The same is true of these hospital-specific talk-groups. It is not the medic unit that is providing the notifications, it is the dispatcher providing the message as an voice announcement (no acknowledgement by, or discussion with, the hospital on these talk-groups).
The medic unit provides the details of the critical nature of their patient to the dispatcher on the medic unit's operations talk-group (north or south medic 'band') and the dispatcher then repeats that same information on the hospital-specific talk-group. I have never heard these used for anyone other than a very critical patient.
These hospital notification talk-groups are definitely in use. On a typical Friday or Saturday night you will hear multiple notifications of critical patients going to Temple Trauma, Einstein Trauma, and Presbyterian Trauma as a result of street crime involving gunshots. In addition, it appears that they are periodically tested during the week. I frequently hear dispatch announce the name of the hospital and then provide a phone number for them to call dispatch to confirm receipt of the test (this is how I recently was able to submit entries for a few hospitals that were missing from the list). These seem to happen most frequency in the 8>10 am timeframe on weekdays.