Yeah, why would you hire a professional web designer when every dork with a WYSIWYG editor and an internet connection (including your weird Uncle Morty) can do it for you?
A couple of reasons spring to mind. First off, I'm not knocking dorks in the least, so some basic definitions are in order here:
Dork- a person of any age who thinks they can design a web site solely because they have the current copy of "SuperWammerdineWebProducerinaBox" software package, when they neither understand the medium, how to adequately apply the software to best exploit it, or care that they lack this knowledge.
Web designer- any individual who, through research, personal experience, time, effort, and perseverance have gained (or are actively gaining) an understanding of what is appropriate for the different styles of web sites that make up the world wide web. A web designer may or may not have formal training or a college degree.
There's a huge portion of the web design world that's younger than some pairs of boots I own, and can design circles around me. They fall into the "web designer" category of this discussion, just as many self-titled "web designers" could equally be titled dorks for the purposes of this article.
So, why not let a dork design your web site? A couple quick reasons, in no particular order:
1. Would you let a dork do your books? Handle your print advertising? Write your business contracts?
2. Most dorks view the world through vastly different eyes than most business people, and many consumers. For dorks, things need to be moving/jumping off the screen or they quickly lose interest. Do you really want your insurance sales site hoppin' and jumpin' all over the screen?
3. I can do it, therefore I should. Really? Are you sure? Suppose your site is a information based site. Is the 17 color background that makes the text (the meat of your site) nearly impossible to read required? Necessary? Even desirable? You decide.
4. "Oh, yeah, that 230kb index page looks fantastic!! Too bad it takes 13 minutes to load over a 28k dialup connection."
5. "Gee Fred, your site looks great, but why does it have a horizontal scroll at 1280 x 1024?"
6. Two words- Flash Animation. Don't get me wrong, Flash has it's place, and done correctly it can really make a web site pop in the readers mind. Unfortunately, most of them fall into category #2 above. Personally, if I'm looking for instructions on how to disassemble my toaster so it'll work when I put it back together I don't want crap flashing all over the screen at me.
There's more, and the list will no doubt be added to as time and energy allow.