Why is tenure at one company automatically assumed to be tenure at one job? I used to work for a major defense contractor. For every engineer there who spent nearly their entire careers in a particular role on a particular program there was another who spent 20+ years moving from program to program, often as a sort of internal consultant. I would honestly put that latter person ahead of both the former and the person who changes companies every 2 - 3 years if I were hiring.
As the parent suggested, I think there is a certain assumption that--even if they've had different roles within a company--they've become so immersed in a particular culture and worldview that they're less valuable than someone who has a more diverse set of experience. That said, if someone has moved around every 2-3 years, especially outside of startup environments, that's something of a flag too.
I think that from the recruiter's point of view, there is an argument for either side. I really think that the employee that has been with multiple companies is often more 'talented' overall but they are usually less loyal.
In software - It's highly desirable for an engineer to be adaptable - Changing companies is a good way to expose yourself to different environments/perspectives (and therefore become adaptable). It's hard to get the same range of experiences from a single company.
This is especially true if you want to become a technical manager/lead (or CTO/CIO) - You need to be exposed to different approaches so that you know what works and what doesn't. You need to have a clear picture of what constitutes a good engineering culture vs a bad one.
I don't think it's always assumed, and I would absolutely agree that someone who has moved around as an internal consultant who may be building things for different parts of the business (some firms might call them SWAT teams or fire jumpers) would be coveted by other firms. It depends on the company.
You reference a 1:1 ratio between engineers in one role and engineers moving around. Based only on my experience I would estimate that ratio closer to 5:1.
I'm sure it depends on the company and how it is organized. We had design groups that handled most hardware and software design. Those engineers only worked on a program until the production design was finalized, at which point they usually went to a different program, sometimes in a completely different city.
We also had a third category, who were the engineers that worked in support activities like manufacturing and failure analysis. They might have the same "job" for years on end, but the job itself changed constantly.
I'd agree -- the outs for a mediocre developer are sticking in the same position, on a legacy product, for years where they won't be noticed, or moving to a new job after they've settled in but before they reach the apex of showing off their skills.
Being able to build a career moving between projects in a large firm, where your reputation is always right next to you, would be quite a bit more challenging for the mediocre developer.
