Yes, I think one of the big union/non-union differences is that there are many additional tasks you can’t be required to take on while an SM. Less staff=more crossover is a factor, and lines do get blurrier with smaller budgets, but AEA means the lines are in fact there, and it’s up to every union member to walk them…or not.
In terms of helping to get SM work, I think knowledge of all aspects of theater is a definite benefit, particularly for non-union work, when most positions have a slash in the job title. I’m very thankful to have held a variety of theater jobs, both during college and professionally. Knowing the challenges other departments or positions have to deal with helps you help them. As Smejs said, you’re better able to troubleshoot in an emergency if you know the language and have a basic grasp of what problems might arise. And I like knowing I’m not asking my crew to do something I haven’t done or wouldn’t do myself. Also, when you’re starting out, you may be able to get board-op or overhire work at a theater where you wouldn’t be hired as an ASM or SM right away. It gives you a way to get your foot in the door, and if you’re on run crew you get to see another SM in action and learn from them, too. But as much as I appreciate my tech background, I’m a Stage Manager because that’s what I do best, and I’m happy to not have to also do jobs I’m less good at.