Really, I don't care about the supposed environmental harm from running R-12. I've seen pictures of military buildings that used Freon fire suppression doing test dumps with 1000lbs+ of HFCs. What's a couple more 12oz cans? Nobody has manufactured this stuff for years. We just keep buying recycled material. Whether my car leaks it out this year or it sits in a bottle a couple more - it's all going into the atmosphere eventually. Even if the use was outright banned, it would keep circulating for a while. So why not use it up? It's really a question of cost.
R-134a is not a friendly material either, nor a particularly good refrigerant. It's just the one that got the nod by politicians. If you really cared about the environment you wouldn't run HFCs anyway. An R134a user looking down on an R-12 user for "green" reasons is like a 300lb guy calling a 315lb guy "fat".
Hydrocarbons cool better, require less refrigerant, cost less, leak less, run at lower compressor pressures and have no impact on the atmosphere. No need to upgrade your system, either. R-134a can burn and we ride around with pressurized gasoline all day, but it seems people are too afraid of a lb of propane/isobutane for it to get popular.