1. You can make money testing and selling all types of cells to finance more expensive equipment or more useful equipment, like a spot welder, solar panels, a better soldering iron, and etc.
2. If a cell leaks, don't do anything ever with it. You remember that post about a Sanyo leaking while charging? Just for giggles, I tried charging it, and it burst on fire. Will be careful next time.
3. Testing 18650s leads to other hobbies, which did cost me money, like high performance flashlights (look up on BLF Q8), but they are still fun to have.
4. With so many 18650s, you aren't going to have to buy any batteries ever again.
5. If I knew lithium-ion was so good to the point of me getting more free time by not shaving as long, I would've gotten in cell salvaging a long time ago