To play devil's advocate here:
In a commercial, or as stated above, lab environment, I would have no problem with this setup.
But in a home, I picture the frustrated homeowner driving over to the big box store and having the following conversation with the "expert":
Harry Homeowner: "Can you help me out? I have a welder and compressor in the garage and they keep tripping the circuit breaker."
Eddie Expert: "What size is the breaker?"
HH: "30 amps."
EE: "No problem. All you need to do is change it to 50 amps. We have those right here."
HH: "I'll take it."
One $25 breaker and possibly a house fire later, problem solved in both counts.
Admittedly, one would have to have a big welder and big compressor running full-tilt at the same time to cause significant heating of the {assumed} #10 feeder. But the repeated stress of overloading can trigger insulation failure down the road.
So, I would prefer to run two circuits just to be sure. If you have to lay pipe or fish NM to the location anyway, upsized pipe or two runs is not a lot more expense or labor than one run or pipe size.
