I recently wrote an article on my personal blog site and that was the exact title of the article. Did you perhaps see that somewhere? Just curious because it was posted in a few places around the web.
Anyway, my feeling is that in many cases it should be spelled properly, but not always. Having it spelled properly can help to make sure that when people type the name in, they will go to the site they intend to go to. If they can spell correctly.
Sometimes though it can be hard to find a domain that is spelled properly that is available for registration, especially if you want a particular extension like .com, .net, etc... In these cases then an improper spelling of a domain name might be the only real option.
Another reason why some people use improperly spelled domain names is to be different, to stand out, to be trendy and so on.
Basically it will all come down to what you are trying to achieve and if you are willing to loose traffic to people spelling your improperly spelled name properly and winding up on someone else's site.
These days the spelling isn't as important as in the past since most people will only go to sites they see a link for.
The other thing is if you use an improper spelling and someone verbally tells someone about your site, they might not tell them the way that you spell it so when they go to type it in, they will again go to the wrong site.
It all comes down to the use and if you are willing to loose a portion of your traffic or not.