Since his comment mentioned that he does not care about weight and thickness then batter cases is a perfect solution for that, it's easier to purchase a battery case then an original replacement batttery
Well, in fairness, I think the original comment was meant more to imply that small changes to make it slightly faster, lighter and thinner weren't that important (but a few iterations of that can add up, so I think it's important, even if I don't always opt for that in my new purchases), not that they didn't matter at all. That said, a battery charger is superior to a replacement battery in that it doesn't require a restart, it may be large enough to recharge more than once, it can charge many types of phones/devices, and it can have a nonstandard shape. It's inferior in some ways as well, since it requires being attached for some period to charge the original phone (which can be impractical or annoying in some circumstances).
Also, another important aspect of a removable battery is that some people like it for privacy. If you can't remove your battery, you can't really assume your phone is truly "off". Even if you can, if you really care about that, you would wait a while to make sure any small internal batteries/transistors have used what power they could hold. Other than that, I'm not sure of a way to ensure the baseband processor isn't on[1].
No. Batteries cost next to nothing. Back in the days I had three additional batteries for my Galaxy S 2 and it was amazing. Others in my team frequently needed time at a charger while I could go from 0% to 100% in a minute while on the go.
The batteries came with a charger and were around 10 Euro each.
So you carried those three additional batteries with you all the time? Honestly how many times a day do you charge your phone? Most phones last full day and I dont have a problem plugging it in at the end of the day to the outlet, instead of carrying a set of batteries with me.
Also, another important aspect of a removable battery is that some people like it for privacy. If you can't remove your battery, you can't really assume your phone is truly "off". Even if you can, if you really care about that, you would wait a while to make sure any small internal batteries/transistors have used what power they could hold. Other than that, I'm not sure of a way to ensure the baseband processor isn't on[1].
1: http://www.osnews.com/story/27416/The_second_operating_syste...