A nice big camera case with lots of pockets and room for every conceivable attachment is great if you're going to a wedding or other event. You'll be able to set your case down somewhere while you're taking pictures. It isn't a problem that it's big, bulky and heavy because you're not going to be carrying it while you take your shots. There are obvious advantages you can pack everything you could possibly need. You'll have all your lenses with you and can include a mini tripod, a flash and extra memory. With the larger camera cases, you are prepared for just about anything.
It's not always feasible, however, to carry one of the many oversized 'pack it all' camera cases. If you're going to be moving quickly from location to location or will be hiking, camping or some other activity, you'll want a smaller camera case. A large case is too cumbersome. You can't be jumping from one spot to another with some monstrous camera bag dragging down on your shoulder.
A small camera bag with padded shoulder straps or one that fastens at your waist is a better choice when you're busy outside. Look for camera cases with easy access so you can reach in and get what you need with one hand. Many of these camera cases feature a holster style that holds your camera and lens and not much more.
Holster style camera cases are ideal when you know you'll only need one camera and an adjustable lens, but it can be limiting. Not having your other lenses, a flash and other gear can mean you might miss a great shot. You'll need to balance your camera needs against the practicality of the bag and your ability to easily transport your equipment and come up with a good compromise.
An alternative that allows you to carry more gear without having to lug around a traditional style camera bag is the stylish camera backpack. These have all the features of the more traditional camera cases, including closed foam dividers for shock absorption, mesh bags for storing batteries and other small items, and neoprene or nylon shells, but they look and pack like a student's backpack.
If you're going to be out and about and want to have more than one camera available or lots of extra lenses, backpack camera cases are a nice compromise. You will have to contend with the inconvenience, however, of stopping what you're doing, removing your backpack and digging around in the case to find what you're looking for. You can even find some good backpack style camera cases with a section for your laptop, making editing on the fly possible.
The next time your favorite shutter bug says, 'Let me grab my camera,' you'll know the reason why he is taking so long is because he's trying to choose between his collection of camera cases for just the right bag.