Tom pretty much hit the nail on the head.
When you reel it up, you just have to get used to moving your thumb back and forth. It takes some practice, but once you get the movement down, its easy.
The reason that some don't have levelwinds is that it causes friction and slows the speed of the reel down when casted. This is for surf casting when they are used for casting long distances.
Another reason that some don't as they are used as a heavy duty reel for offshore use, is that alot of levelwinds can't take the heavy pressure put on them from some of the bigger fish off shore when they pull the drag out fast. Ive seen some of the cheaper 4/0 reels that have a levelwind, and only the cheap ones have a levelwind in that style reel. Ive seen a brand new reel like that become worthless after one king mackeral hits the line. Stripped the levelwind and locked it up, and because the levelwind locked up, it couldn't let line out fast enough to keep up with the fish and broke off.