Calling it a total waste is way off. That liner saves you from scrubbing baked-on grease every single time. It turns a messy chore into just tossing something in the dishwasher.
Read an article that said those liners actually mess with the heat flow in some ovens, so you end up with uneven cooking. But honestly, if you're just doing basic stuff like frozen pizzas or casseroles, it probably doesn't matter that much. I've seen a few people online swear by them for keeping the bottom clean, especially in old apartment ovens that are already a mess. Sounds like yours worked fine, so that's all that really counts.