I definitely think battery cases are worthwhile & love my Mophie case for my Samsung Galaxy S6. I use it everyday for that extra phone charge when I don’t want to be plugged into something or its really inconvenient to be.
I love my PowerCore 20,100 for when I’m staying in one spot for an extended period at home (or taking out with me when I need a lot of extra juice), but sometimes its not practical for certain things. Especially when I’m around the house cleaning, or doing some things that require me to move stuff for an extended period, its really not feasible to have my heavy PowerCore in my pocket plugged into my phone.
It just gets in the way, & I risk having them fall out of my pocket bending down or having the cable liable to snag on stuff. So battery cases come in huge for me with that since I can just switch my regular Otterbox case & put on my Mophie case when I need to & have no cables to worry about.
It also provides good solid protection as well, I’ve dropped my Mophie case a couple of times on my kitchen tile, & the case as well as my phone are just fine.
As for it bring large & heavy, while there is some weight & slight bulk to the Mophie charging case I consider it a fair trade. Especially since its quality made & serves as a a pretty good protective case as well as a charging one. And I quickly got used to it. Also, its much lighter than carrying my Powercore in my pocket.