This is blog post worthy, as I think this idea could fleshed out. Here are some questions in no particular order of importance, just curious (and showing some evidence this could be fleshed out):

1. How many times did you try it?

2. Why do you think it works?

3. How do you find companies that have both technologies?

4. Have you seen a difference in the willingness to let you learn the technology that you'd want to learn?

5. Do you know of times when this approach failed, if so why?

6. Are you of the opinion that if you'd spend 5 years working on JS for 100% of the time vs 5 years 50% of the time and 5 years 50% of the time on a technology that you'd like, that you would be as experienced in JS in both cases? Even if you're not, could people prove that you're not? Should you just say you've had 5 years of XP with JS?

7. Are there moments when this approach is strictly worse compared to a traditional approach despite it not failing, if so why?

8. Are there strategies that synergize well with this approach? Are there strategies that diminish the value of this approach (aka work against it somehow)?

Alright, brainstorm time is over. I'd read your blog post about it, or having an email discussion would be awesome too (my email is in my profile, if you think it'd be fun).