When I talk about investing in a set bag I don't mean as a cool way to carry your kit around. I mean as a tool to use on set, use it for the intended purpose.
The purpose of a set bag is to carry "on set" the makeup and tools that are required for touch-ups. You use it to carry the items you need to do touch-ups and the size of the bag will depend on the scale of the jobs you're doing. I personally have two (I will be adding a third soon), it's about the practicalities of working.
My Medium bag (from Screenface) is used when I am on a film set and need a little more space to carry kit onto the set.
In the next few weeks, I am planning to add the Mykitco backstage buddy, the reasoning being it will be practical for my next two films, yes I will take my small bag with me for my assistant to use.
Practicalities being that I can carry the items I need for touch-ups such as powder, brushes and a palette, perhaps even glue. I will not, however, use it to carry the full kit on a train or a plane. It's ostentatious and showy, I am not interested in explaining my kit as I travel. I want it stored in a case so I can just pick what I need without having to unpack everything so I can have the things I need on hand. I will use it as intended.
If you are all about the show and your clients are of the type that needs it, then go for it. For me it's about practicalities not showing off.