It is a bit more expensive than the substitutes and requires a EPA 609 certificate (easy to get certified) but here are the reasons I would put forth:

1. The R-12 substitutes like freeze 12 and especially the hydrocarbon formulations are composed of multiple substances and have a tendency to fractionate over time which changes the pressure/temperature characteristics if you have a small leak.
2. Any refrigerant other than r12 or R134 is not mainstream in the sense that A/C repair shops do not generally use alternative refrigerants and may not service systems that are charged with them
3. Using R12 means you do not have to flush or change out any of the original components
4. The system was designed to use R12

Do some due diligence and especially do not take any of the marketing boilerplate from the alternatives at face value.

If you do choose to charge with the R-12 alternatives, be sure to have an A/C shop evacuate the remaining R-12 and not add the alternative to make up the charge.