If I'm working in an industry I'm leaving then eight hours of work getting better at, say, C and C++ and embedded programming and low-level multimedia work is eight hours that will do me no good at my next job, or getting a next job. So the net benefit in job-hunting terms (rather than straight-up skill) is zero. The straight-up skill effect is good, but not 100% transferrable. So overall, eight hours on the job in a not-really-related industry is pretty low value.
In less mature web technologies (Rails, but also a lot of other related bits), I'm losing time by having no demonstratable experience during the time it exists. If I say at an interview "Rails has only existed for five years, so of course I have no professional experience in it", that's dubious but approximately reasonable. If I say the same thing and substitute "ten years", it's outright foolish -- my lack of experience looks seriously bad rather than basically understandable.
The five intervening years are thus a day by day slide from "excusable" to "inexcusable".
Worse, in a young technology I can be among the better people pretty quickly because nobody has much experience, so I can turn effort into seniority quickly. When a good technology has been around ten years, there are a lot of highly skilled five-year-plus veterans to compete with. So the five intervening years are also a slow slide from being perceived as "semi-experienced but obviously skilled in a new technology" to being perceived as "inexperienced but smart and able to learn" at the same level of skill. That's fine for a junior engineer, and inexcusable for a senior engineer.
So eight hours working at ACCESS means I gain a chunk of low-value experience and lose a chunk of mid-value credibility and seniority in the industry I want to get into. It's a small loss, but a loss. I can supplement that by working hard on Rails in my spare time -- that work gains me the credibility, seniority and skill, but it takes extra time and effort. If I don't supplement with spare time and effort, I am instead slowly losing ground.
That's also why I'm intentionally trying to jump into a relatively new technology. Jumping from one old, established technology into another is even harder, because the competition is with more senior people, and the supply of competitors is well established. Rails is still young enough that it's rare to have three years of experience and they're scrambling for people who can do it.