Lou,
I can't make you see the large operational and functional differences between the units, which is fine. If you choose a product based strictly on economic factors and the ability to DIY and strictly control it without a "thrid party" entity, so be it. You chose the C3 as suitable for your purposes, which may or may not have been a truly informed decision based on percieved value vs. risk or return on investment.
I'll assume that since you're not in the trade, you don't have access or have really seen the real programming and functions contained on the units I deal with, Uplink, Telguard, AES, Conn24 and Alarmnet. All of them have provisions for supervising power, communications and multiple other items independent of the host panel's connection. The host panel really only supervises it's connection to the unit and gets a output from the cell to notify the end user of any potential issues, that's it. All the other supervisiories are directed to the central station to be acted upon, because as you put it, what are you going to do if X is down/fails/what have you....but then again, with a C3 itself, you know none of that and then to do such, you need to homebrew your own solution (hack, if you will) for items that can be supervised by means within your control. Personally, I'd rather have the CS inform Mr. Jones that they're having communication issues or a low battery in their cell rather than discovering such from a keypad beeping or display.
The item that you're missing or is getting lost in your translation is the monitoring aspect and really how much should be supervised and monitored. We're not talking big brother or being unreasonable. Sure, you can't fix a cell tower or battery/AC loss, etc. remotely, but at least the central station is informing you of an issue prior to it becoming a mission critical failure, or at least informing you that it is a mission critical failure that should be addressed ASAP. I got many of those calls with the last hurricane and storm that went through, affecting upwards of 30% of every cell tower in the NE United States.
The C3 is a glamourized cell phone...that's all. You have no way of knowing anything about it's functionality until it's too late. Period. As long as it's putting voltage out on the simulated telephone lines, the connected panel is going to remain happy.When does the C3 test itself, the power supply, the battery, the comms route? How is the C3 able to report anything other than what the panel detects and is able to report, assuming that it's still connected to the panel?
In the specific case of the 2500, I install a separate power supply to meet the 2.5A output that is needed, not the host panel, and when you source the separate power supply, wouldn't it be irresponsible to install a power supply that doesn't have AC and DC supervision connections as well as battery supervision, let alone not connect those to the host panel? Wouldn't it be just as irresponsible to not enable reporting of all the internal supervisiories the unit can transmit?
Simply put, the C3 may do 70% of what a real communicator does, but the fact remains that the 30% of what it does not do is very significant, no matter how much you choose to discount them.