In 2014, I wouldn’t consider works you license (ebooks and similar) truly optional. If you want a secure income, they’re essential.
Yes, it’s a challenge to find time to create ebooks. However, most writers leave money on the table, both when they write fiction, and when they write nonfiction. I discussed Draft2Digital adding Scribd to their distribution. You need to distribute as widely as possible. However, considering your formats is essential too.