Two days ago, I posted on the reasons why you should have a pseudonym. Now let's talk about why you may not need one.
"But, Mark, my readers will get confused if I start writing in other genres."
Hogwash! Readers are not stupid, and may even like it when their favorite author writes in another genre. I just got done reading the latest John Grisham legal thriller (The Racketeer), and I have to say that it was a lot better than many of legal thrillers he's been publishing over the past few years. But here's the funny part: my favorite books by Grisham are not the legal thrillers (okay, maybe a little bit) but the stories that outside the genre: Skipping Christmas, A Painted House, and Bleachers. The same goes with Stephen King. My favorites from him are The Green Mile and On Writing.
This seems to be the main reason why publishers want authors to write under a pseudonym. Dean Koontz wrote several stories under a pseudonym, and has since becoming a mega-bestseller republished those books under his own name.
"Okay, Mark, I get it. But I'm not Stephen King or Rowling or Koontz. I'm just (insert your name here) from (insert place of residence)."
You make a valid point. However, all of the writers you named above started as a nobody. They, through sheer dogged determination, churned out books that connected with people en mass. You can do this to. Just sit your ass in the chair and write. That's all they do.
Here's my final argument for why you don't need (or may not need) a pseudonym. Why do you really think you need to? Years ago, I wrote several of my novels under a pseudonym--name to be withheld at this time--but then came to the realization that I may not have to, and if a publisher forced me to do so in order to publish a book, then I'll consider it. For now, I write under my own name.
I want to repeat this again: readers are NOT stupid. They may also appreciate it when you do branch out into other genres. Let me leave you with this final thought (okay, I said this above, but now I'm serious): who is Elvis Presley? He's the King of Rock and Roll! But did he sing just rock and roll? No. The reason he was--and still is--considered the King was because he branched out into so many other musical genres, including hymnals and folk, that he captured the lovers of those musical tastes and drew them back in to his rock and roll.
Elvis was a master.
Follow what the masters have done.
And remember: readers are NOT stupid. They just may not be able to find you if you happen to write under a pseudonym in another genre, and you could've expanded their reading tastes. Not long ago, I read a collection of science fiction/western short stories by Alan Dean Foster. I'm not a reader of westerns, but after that I may branch out my reading tastes to include the likes of Zane Grey and Louis Lamour.