Amex Platinum Charge Insurance here. As stated elsewhere, hire car excess (and the "value" offered by the hire car company fees for reduction) are a major reason to consider domestic TI, but cancellation and other costs are also a significant consideration. May not be much of a concern if you are simply doing a 1 or 2 day business trip, and can afford to pay a last minute fare if need be, but a whole different story if you have booked a 2 week holiday and had to pre-pay a lot of accommodation etc.
Prior to the Amex Platinum, I would have been in the "consider each trip and evaluate the risks" category - wouldn't bother for a 1 - 2 day work trip BNE-SYD, using a cab/train etc, but would (and did) for a 2 week holiday around Tassie over the Xmas - NY period (what percentage of Tasmanian cars are hire cars? - seems like at least 50% at that time of year)