Not much different than taking the shampoo or other "consumable" room amenity, IMHO. Since you needed it, I'm certain your hotel would be glad that you could use this on the rest of your trip. That's why they call it the hospitality industry.

I once asked a maid if I could have one from her cart for a long drive I was headed out on. She was happy to give it to me.

I guess you have to ask if the hotel would have charged you for the item if they knew you took it. In this case the answer is almost certainly "no".

If you want to feel guilt free, next time just call the front desk and tell them you need the darn thing and ask if you can take it. But frankly I think they'll think you're a bit overboard to even ask on this item!