Keep in mind there are different grades of awning fabrics and if you get a topper that's entry level it's only got about 4-5 years in frequent exposure before the UV and ozone begin to deteriorate it.  Personally, I will absolutely never got without toppers on my slides because of all the pros of shedding water, protecting the roofing and top seal on the slide, and providing the thermal break.  If you get a quality topper that is properly hung (that's key) you won't have to fuss with bellying, flapping, or it holding water.  With regard to properly hanging it, if it doesn't come direct from the factory installed, I install it myself.  I have yet to find a single dealer service department install one properly so that it has proper tension across the whole face because they nearly all make the same exact installation mistake - following a body line versus making absolutely certain to parallel the slide-out.