Considering the joy-con lasts 20 hours at full charge and you charge them by sliding them into the console, I feel like these are useless unless you really forget to slide them into the switch all the time or are playing more than 20 hours at a time
I feel like the only pro to this is that they would make the joycons larger and more comfortable to hold. Other than that, having a battery back for them seems useless.
I got them for $15 and although they increase the bulkiness of the joycons they sre slightly too big (even though I have huge hands); also you have to take them off if you want to attaches your joy cons to your switch (if that wasnt obvious).