Yes and no. The system is limited by the amount of available frequencies. Therefore, allowing lets say a City of Newburgh PD Dispatch talkgroup to be repeated off the Arden tower "full-time" would cause one of those frequencies to be tied up for no reason other than your listening pleasure. By doing this, you just lost one frequncy on the Arden system. Add Cornwall, New Windsor, Town of Newburgh and then the Woodbury PD unit tries to transmit and gets a busy signal on his home system. This is why the FIRE 800 talkgroup used by Middlehope FD and some Coordinators is NOT linked to the 46.160 low band fire channel. Add a storm to this, downed trees and wires and everyone gets a busy signal. Listen to the EMS 718 frequency and how busy it can get. Now picture that being repeated full-time off Graham EDACS, which only has 3 available frequencies, one of which is the control channel. Also think...when was the last time you saw a City of Newburgh PD unit in Deerpark? Does that talkgroup really need to be heard out there?
Certain talkgroups would be fully linked. One example is the Sheriff's Patrol talkgroup. Like TRAUMA74 stated, a unit on one system on one side of the county cannot talk to a unit on the other side of the county unless he switches to that system. Additionally, as the SO unit drives around the county, he has to manually switch systems. We all know how dangerous this can be when you are out of the car on a traffic stop and get the "unable to copy on portable, switch towers or change your position" from the dispatcher, especially if you are on the ground fighting with someone calling for help and hear that because you forgot to switch systems on your portable when you got out of the car. PD's like Blooming Grove who dumped the EDACS for a stand-alone VHF repeater would have to use three separate sites just to cover their town (Beacon, Arden, Sam's Point). In other words, units in the same TOWN couldn't talk to each other on portable in their respective zones without linking. This would help them.
Other examples of linked TG's are the OC 911 polling channel, Intersystems, POLICE 800 and a few others. This would allow for full "countywide interop" when needed. Other uses include a unit like Cornwall the ability to talk back to the dispatcher if lets say he gets into a vehicle pursuit that goes into Woodbury. Cornwall only has Beacon in their radios, so the ability to switch over to a linked talkgroup would allow him to operate off Arden, while dispatch and other units are still on Beacon.
Now if they do it right (and we all know history there), they should set it up similar to Motorola Smartzone systems. If a unit travels to another area, he would affiliate to that system and THEN his talkgroup would appear on that system and be linked back to his home system. We'll use CNPD as an example again. Chief goes to a meeting in Mt. Hope...as he travels his radio switches to the Graham tower. His PATROL talkgroup now appears on that tower and he can hear his units operating off Beacon. He transmits off Graham and is heard on Beacon. His Lieutenant is attending a meeting in Woodbury. As he travels south, his radio affiliates to Arden. He can now hear his units on Beacon and can talk to them off Arden. He can also talk to his Chief, who is hearing him on Graham.
Hope this makes sense. I also attended various interop meetings, but have been out of the loop for a little bit. I do have Unitrunker up and running and can confirm what pro92b posted as far as system ID, channels and LCN's. I also have pretty much every talkgroup and LID number ID'd on the system, so I'll be able to see what develops.