Just don't plan on using a USB mic with locked timing ref lol.
There's much to be gained from using a real loopback timing ref, I think there is no question why other software (REW, ARTA, SE) has moved in this direction.
So does using a loopback provide any real time timing controls when doing impulse measurements? From a tutorial I looked at on part express tech fourm, it seems that loopback is not a dynamic feature, it seems more for the ability to create a static calibration file using the make button under calibrations.
That's correct, loopback cal "make" in Holm just creates frequency response calibration for your equipment, nothing to do with timing reference. It's the same as a "soundcard calibration" in REW. Optional process for the most part as the FR compensation only occurs at the very frequency extremes, calibration can be done once, as long as you use the same equipment for measurement all the time. Perhaps for some cheap class-D amp with pre-filter feedback, calibration should be done for every speaker connected, but better to avoid using non-linear amplifiers for measurement unless for permanent installation like a plate amp.