To put it more into perspective (along with the Batboard thread)
On a 3600 system, the radio does not need to affliate to hear the conversations. However, you still need the system key (a software file or hardware dongle) to program in the trunked system information. Motorola has considered the Software Key as IP property/copyrighted (if someone really wanted to push the issue - how far it would go, I dunno).
If not programmed correctly, the radio will attempt to affliate to the system. In most states, this alone is considered a computer crime as the radio is accessing a computer system (which the controller is) without authorization, along with any other related charges. You don't need to press the PTT for this to happen.
Another danger is if the system manager is proactive in monitoring ID's that are not authorized, or runs reports that shows ID's affilating to multiple sites or talkgroups. Even if your radio at the moment is not setup to transmit, if the system manager (or automated software that is available) sends a kill command to that ID, your radio will go dark and you have yourself a nice door stop.
9600 systems are a bit more tricky as the P25 standard REQUIRES affilation in order to receive the talkgroup audio. There are certain radios out there that do not require this, but the Motorola ones do. There are the hidden talkgroup modes and some other tricks that requries some thought processes, but generally not worth the aggravation.
One of them is here:
Scanning a trunking network with a ASTRO25 Radio V2 - YouTube
In addtion, your still only limited to a few talkgroups at a time. Current scanners are just as cheap and do not require $70 batteries and chargers for consumables.