Sadly, this "learn one thing well, first" problem isn't limited solely to (would be) technical practitioners.
Company I work for has been trying to grow to meet client demands. Due to both available rates and available talent, they've started to hire junior staff to fill positions. Unfortunately, many of the managers don't seem to understand how to use those juniors. Instead of instructing, "learn this one technology inside-and-out, then use it as your point of reference for learning the other technologies we need you to know," they seem to fall into the managerial-equivalent "too many tools to learn". Basically, they try to bounce these new people across all the tools, never affording them the opportunity to build themselves a Rosetta Stone.
I get bounced across a lot of projects because I long ago built myself a very good Rosetta Stone. So, I find myself having to back-channel to these managers (and to some of our customers, as well), "that's not how you senior-up your juniors, that's how you frustrate and burn them out: even if some of them manage to learn anything to a useful degree to you, they're going to leave as soon as they think they've fattened their resume enough."
You should be the one to be in charge of such juniors, or at least to get more in the loop...
We're a place where coders share, stay up-to-date and grow their careers.
We strive for transparency and don't collect excess data.