Oh nothing stops you from having toy stuff around! I have a lot of stuff like that, but you don't spend 6 months on it, you spend an afternoon here and there on it..
Sometime it's worth 'gambling' too -- for example in 2012 I went all 3d-printer-y, learned everything firmware, hardware, design tools etc -- my justification was a bit 'thin' as in "perhaps it's the new 'micro computer' generation and I'd better be on it" -- 9 months later I decided it wasn't, and dropped it mostly.
Then 2 years later I landed a HUGE paying job completely related to that tech, completely out of the blue, and that more than repaid the time I spent on it.
For me, I actually feel passionate about tech choices because some tech is just awesome and fun. Most of the enterprise shit is not fun at all, but very sellable.
But as a junior Dev, I would start with sellable of course.