It really depends on where/when you camp.
I have personally watched others struggle with them in high winds, and in situations where leaves etc. found their way under them.
Though I have never seen this, I bet snow would be problematic as well.
If you are a fair weather camper, who sets up for short weekend stays in wooded areas when high winds are unlikely,,,, then toppers make sense.
The more of the factors I listed impact your style of camping, the less they make sense.
Choose wisely.
Now if they made toppers that one could choose to deploy or not based on prevailing conditions.... That would be a no brainer.
I have never seen or heard of such a topper though.