Many think they need to move from the kit lens to an "L" lens (Canon) or the Nikor Professional line of lenses (Nikon). If you are going for improved images, stop! Your kit lens can produce wonderful, crisp, and sharp photos if you take the time to learn how to use it. However, if you are going to take your equipment out in weather extremes, jostle it around on many trips, or use it 7 days a week and 8 plus hours per day, go for the pro lenses. Or if you've just won the Lotto, indulge yourself with a professional lens.
Pure and simple, the professional lenses are designed for durability and producing images under extremes. They are not designed to produce museum quality images. The photographer alone can do that with any lens. They just need to know how to go from looking to seeing and the need to understand how to use their current equipment (Yes, folks, even the cheap stuff).
Another consideration, do not be afraid of the third party lenses. Tamron, Sigma, and Tokina produce some wonderfully sharp lenses at a fraction of the cost of Canon and Nikon professional series.
Unless your are a professional making a living off your photography or you work in very adverse conditions, you do not need to spend the extra on a professional lens. If you want to play with a professional lens or you have an event that meets the criteria for a pro lens, look into renting one from a local rental place or online rental site.