I see absolutely no reason not to completely unplug the cellular modem. The only thing that would stop me is an annoying error message or warning light in the gauge cluster. My car does not display any of these, but unplugging the modem results in losing the right speaker and microphone, unless a bypass harness is used.
Wouldn't it be better to connect resistive pigtails to the antenna connectors on the board? A little more work to get to, but less risk to damaging paint and weather seals, and would do a better job preventing signal leakage. I'm no expert on such things, but will definitely be looking at something like that for the next car I buy.
Not sure what you mean, the antennas are on the sharkfin board and you get to it from under the headliner not from the top of the car. It's much easier just to disconnect the cable that goes to the sharkfin than actually removing the entire module in the sharkfin.
The modem is usually in the sharkfin with the XM radio chipset and GPS. If you can unplug it at the sharkfin that's usually the best course of action. Some cars may bark at you, but mine just says it can't detect GPS if I attempt to use it (which I never use anyway).