While learning about your camera is the best way to improve your picture quality, and disecting compositions of shots you like is the best way to improve your photography, there are vast and severe limitations on any DSLR kit lens. They are
terrible. Better glass will give you much sharper pictures with very little effort on the part of the person behind the camera, but you're right. An L series is not for someone who is just beginning. Or even someone who's been at it for more than a year and a half, but for someone who has reached his camera/lens potential and wants to take the next step. I feel I have outgrown my XTi body, and am looking forward to upgrading to a 7D when it comes out (based on ISO performance), as well as the limitations when shooting with a Prime (even a fast one) for concert shots for PP, or even every day.
I am NOT making any profit on my photography (
yet), though I have been published in print a few times, and I felt buying the L series was something I needed to do to improve my skillset. I absolutely do not advocate everyone going out and buying a lens this expensive, that's for sure. In fact, I originally wanted a 70-200mm f/2.8L ($1700) really, really bad. During a recent trip to San Francisco, I knew I'd have some good photo ops, and wasn't thrilled with taking just the 50mm prime, the (terrible) 18-55kit lens and my 55-250 IS, so I decided to RENT the 70-200, and the 24-70 caught my eye, so I picked that up too. Nearly the entire trip, I used (and fell in love with) the 24-70, and the functionality and use of the 70-200 was nearly non-existant. Total price for a week with both lenses, $150. NOT dropping $1700 on a lens I would have been extremely dissapointed in, and finding one I absolutely loved was worth every single penny.
Moral of the story: Learn your camera. Once you feel you need to advance, RENT before you buy.