You are right, a writer can join an ongoing series for a planned out arc, but I think this can sometimes make ongoing series disjointed. Even when it is done well, the writer is confined to the starting point they were given. I like that in limited series writers can do something crazy with no consequence. They aren't limited to the story as told so far, or leaving the story open to continue. It is similar to "What if?" comics in Marvel, you can completely change what motivates a character, kill people off, etc and it has no effect on the main continuity.
Limited series are also nice for getting interested in new characters. As you said, it is hard to start reading an ongoing series for someone you aren't familiar with. You can read a self-contained story and if you like the character, seek out more of the ongoing stories.
On the other hand, ongoing series have the opportunity to go much deeper into characters. You can just tell more story in hundreds of issues than a dozen or less.