It's true that publishers like series. If readers invest in characters, they like to know more. I wonder if it's easier to get a book deal with a potential series than a collection of one-off books with unique characters.
I was in a writing group the other day listening to a rather good historical writer read out an extract from a book he was writing. He had been unable to form all his ideas into his first novel, and had already planned two sequels. This wasn't cynical, he isn't yet looking at publishers, but for him the appeal of a series of books about well developed characters was as much the writer's as the reader's. It occurred to me that it's so easy to write a book when you already know the people, the relationships, and have had time to consider the longer story arcs.
But. In writing the first book, you set some things in stone, and it's difficult to go back and change them. I also find it hard to keep tabs on everyone and all the back story. I should keep records, obviously, but I don't because I always think I'll remember it. Now, writing book 2 and 3 and planning books 4 and 5...I'm running into continuity errors. Big ones.
The only thing that's helping is having to edit the older books while I'm writing the new books, that has saved me from a few serious slip ups. There's a lot of going backwards and forwards, checking, checking.
A friend of mine is just hoping to sell her book, but looking at creating a series possibility to make it more saleable. That's probably good advice for anyone wanting to sell a book in this market. At least if they make good sales the publishers will encourage more in the same vein, so it helps to have outlines planned even if just in your head. Even if you never write them. But, who knows? You might fall in love with your characters and want to take them on more adventures.
Oh, and in case you missed it, I'm blogging for Mslexia at the moment!