Y.N.
That private play time is significant to her development. This is not only great for her independence, but also her intellectual development. It is during self engrossed play and exploration that we all learn. It's no different than when an adult gets a new software program or stereo and just fidgets with it for hours to understand it. She's just showing off her intelligence and love for learning!
Also, your child's ability and comfort with playing off by herself is indicative of the bond the two of you have. This may seem contradictory, but children who are secure that mom is there are often less fearful about leaving mom for a while.
So be proud of your bond with your little genius and make use of that free time!