It depends on the insulation value of the individual slide, if the topper will help keep the slide cooler or not, because that is one of the advantages of a topper.
If you plan to use in winter, then do not get one because you will have to scrape snow/ice off before you can close it.
Same goes for summer use, if debris gathers under the topper it more difficult to remove it in order to close it.
If a heavy rain cause the topper to pool, then it might stretch to the point of no return. It's also something you need to clean just like an awning.
I always carried a small ladder and broom. It took a few minutes to sweep the slide in order to close it.
Basically, it's up to you . If you have experience with a topper already as you stated, then only you know if it's worth having to spend $600 for one. :)