Yes, because a regular knife just won't do the job:


I didn't know there was a special tool for cutting avocados....but looking at the picture, it appears to be made of plastic.
I use a regular paring knife when cutting them, but I always use a glass bowl to make guacamole. Avocados turn dark once they are cut, and for some reason, contact with metal speeds up the process.
I suppose using a plastic cutter, would also help to delay the process, but since I don't cut one until I'm going to use it, an avocado cutter is not something I'm going to spend money on.