IIRC, even most (all?) software engineers working for Boeing in Washington State are required to join the union.

I worked for Boeing in Colorado on a major govt satellite program several years ago (non-union in CO of course), and when I received a WARN notice when our contract was ending I began looking around the company for other positions. All the SW Eng jobs I could find in the commercial aviation BU in Washington were listed as being "Union." Blech.

I ended up leaving the company after I figured out that Boeing's hiring and transfer policies meant it was easier to quit and apply for jobs as an external applicant, rather than applying as an internal candidate.

I don't know if it's any different for software engineers, but I have a friend who did an industrial engineering internship there a couple years ago. He was treated exactly like a full-time employee (full benefits, 401k, etc.) and had the option to join the union or not. It was barely any more for him to join so he did.

False. As an engineer you have the option of joining SPEEA, but it is not a requirement.

Joining the union offers some benefits, and some people do so for personal/political reasons.

One useful benefit was publication on anonymous salary information around raise time. You see exactly where you stand without violating the privacy of your peers.

Source: I'm a former Boeing employee (DS9 - Real-time systems simulation engineer)

Right. But if you don't join, you still have to pay union dues, right? That requirement kind of blurs things in my mind.