I suppose those of us who have never had them continue to live without them and we're getting along just fine. Those who do have them seem to really like them. And then there are those who have problems with them, billowing in the wind, getting dirt under them are the two biggest offenses.
I suppose, someday, I'll get to the point where I won't be able to climb a ladder any more to brush off the slide out tops. When that day comes, I may be a little more open minded to getting the slide toppers.
For the most part, I think they do help to keep things like leaves and stuff off. But there is still the problem of stuff blowing under them too. And then if you retract the slides, there's the risk that any hard debrie, like small sticks or whatever, could punch out and damage the slide seals. For me, it's not worth the risk (yet) ... but maybe someday my ideas will change.