My family once accompanied me to a conference, and we took our then-3-year-old daughter to an outdoor aquarium before one of my meetings. She spent most of the morning talking to imaginary friends, and at one point she asked us for some space because she and her friends needed to have a "meeting." Even as a creativity researcher, the level of imaginary play worried me a bit, and I shared my concern with a colleague when I eventually made it to my meeting. It turned out that I was sitting right behind two of the world's leading scholars on early childhood play - Jerry Singer from Yale and Sandy Russ from Case Western - and as they overheard my concerns, they both spun around in their chairs and gave me an earful! I'll never forget Jerry's first comment, "It's a good thing, you should encourage more of it, and you'll be grateful she's doing it." It was, we did, and we are.
As someone who studies education policy and practices in other countries, I have always found it interesting that those countries that we tend to compare ourselves to (Finland, Japan, China) often have play-based early childhood education programs. Academic content tends to be woven into unstructured and structured play. These countries appear to have decided that many "21st century skills" (creativity, collaboration, communication, critical thinking; see P21. org) are best taught in young children through play.
I also recommend Sandy's latest book on the topic, Pretend Play in Childhood: Foundation of Adult Creativity (published by the American Psychological Association).