If the base is not supported well, over time with use, with each flex, you are liable to fatigue the glass fibers and allow it to flex more and more. Eventually, it could fail but prior to that it will start to feel softer (since it won't have as much strength), and micro fractures start to show up that will look like it is getting dirty (well, it is - dirt is getting into the microfractures). If this ever gets extreme, it can cause the seal to fail or it will crack all the way through. It is a really good idea to support the base well during the install with something. Ever leaned against a corvette's door or fender? It will only oilcan so many times before the paint fails (like the gell-coat on your shower). the better ones are well supported, but it still doesn't hurt to support it well in the gaps.