I've built leads just like that, minus the double-ended contraption.
In a working shop, they are no good for the reasons already stated. Without a proper cable grip on the box, the wire ends WILL pull out of the back of the receptacle. I had one go overnight for no reason at all. Examined it the next morning, an entire conductor had come undone. I figure it got loose during the day and the stiffness of the wire itself was it's undoing in the night. But it only takes one individual strand. Those boxes also admit quite a bit of grinding dust and slag.
And those plastic male plugs are not without flaw either, if the built in cable clamp is cheesy plastic, the cable will eventually loosen and then it's only being held in by the set screws holding the conductors. The individual strands begin to fray, and then one day when you go to pull the plug, pop!
Been there, done that. Pass.
In industry we do it because it's expedient, and we can get away with it because your typical industrial environment is designed to tolerate such laziness. The wiring is large gauge, in metal conduit. The buildings are usually metal and intended to contain chaos such as welding, and the work spaces are usually clear of immediate fire hazards.
A typical home garage has none of those inherent advantages and will not tolerate much with regards to fire and electricity.
Would it be okay for a home shop? Maybe.
Do people do it and not have any problems? Sure.
Would I do it in my own home shop? No. If ever something DOES happen, any homeowners ins. will be void the second it's discovered. No thanks.
What do I use? Hubble twist locks. Yes, expensive. But you can get generic 30A twist locks for about the same money as the bladed connectors and have a connection that is better in multitude of ways.
