>>since the Moto X battery is non-removable, your example is worthless<<
Not really. If the Incipio is internal, then there is no choice: you need to carry it all the time. Since you agree that it's external, then we do have choices. You can choose to have it attached all the time for the occasional use (for me it would be more than 10 hours, which makes it unnecessary). Or you can choose to use the Moto X without the extra burden, and get a separate battery charger for the occasional use. I have 5-6 power banks, some smaller than a cigarette case, weight just a few ozs and cost around $10. It is not hard to find a small external battery.
>>the Incipio case was PERFECT for my day on the boat on Thursday. I was able to keep the phone+case in my jacket pocket<<
You just proved my point. Since you carry a jacket with pockets, what's preventing you from using the Moto X without extra burden, and have a separate small charger in a jacket pocket "just in case"? I guess people can say something like "I may lose the charger (but not the Moto X?), or "I feel Moto X is too light without the extra weight", or even "I like to punish myself (sorry I have to put that in)", etc. But really, the only scenario I can think of to support the use of an Incipio would be if I am allowed to wear only a swim trunk for the whole day, with zero or one small pocket, and am not allowed to carry any jacket or bag. I don't think that happens to anybody that often. And frankly if you are in a swim trunk, you probably won't be doing heaving computing anyway (unless you are a marine biologist), and can always recharge the Moto X at the end of your activity.