At least for me, learning an API (I'm assuming that "in-depth" is implied) that I don't use is a waste of time. I'd rather spend my limited amount of time and brain power learning new concepts or exploring new tools than becoming intimately familiar with an existing tool that I don't need to use now. When I need a particular tool that I don't have or have to use a tool that I'm not familiar with, that's the time to learn it in some depth. Before that I might do enough investigation to know whether it is going to be useful to me or not, but not much more.
