I use an early '00s TI calculator daily. It's great for quick calculations that are a bit beyond my mental math capabilities.

I've considered these types of mods before. The biggest reason I haven't is the ROI feels impossibly low. A set of 4 disposal batteries last >6 months. A lithium ion battery lasts ~4 years before needing to be replaced. Doing a lithium ion mod would save me less than 32 disposable batteries before I would need to replace the rechargeable battery. It seems way way faster to just keep some disposable batteries in my desk, and not have to worry about charging or replacing a lithium pack. This is cool, but feels impractical.