I still rent the router/modem from my ISP. Looking back over time and the amount paid it's certainly easy to see that it's been an unwise choice financially but if there was failures or other issues with it, it could have been a wise choice. Overall the cost is minimal and the amount of extra money I would get in my pocket is in the noise floor of my salary.

If my goal was to examine every aspect of my spending to be as thrifty as possible there are many other things that would provide a greater savings than this. Ultimately for this one the cost to convince if there are issues is just worth it. I suspect for many people it's the same thing.