In the whole scheme of things, a few pairs of quality/trusted batteries are inexpensive compared to other vape equipment, especially if you buy premade juice.
I would harvest batteries only as a last resort. Even a 'free' battery pack wouldn't entice me to harvest the cells unless I was unable to purchase new reliable cells like we can now. Just no reason to take chances when we have the option to buy them new/individually.
The Sanyo 20700B's I've been using for the past year are still performing very well. I recently added two new ones to the rotation, and I honestly can't tell that much difference between the new/old cells. Even replacing batteries yearly (if you only use a few daily) is rather inexpensive. If you have many sets and rotate through them, they should last quite a while. Just not worth it to scrimp on batteries. Skip Starbucks for a few days and get some fresh cells from a reliable vendor.