Since G transit visas are pretty much the same price as L tourist visas and take the same amount of hassle to get, it's a no-brainer to just get the L visa, if one is getting a visa.
The main reason for G's to still exist is that some "questionable" nationalities cannot get regular L or other types of visas without a sponsor or joining a group tour, and if they need to pass through China as an individual traveler, a G is the most practical legal way they are allowed to do it. This situation doesn't apply to most nationalities though.