Improve battery life
With the official Google Play app disabled, your phone's battery lasts longer.
Curiosity may kill the cat, but in case of Android, Google’s curiosity also kills the battery.
Google wants to know everything about you, including what you do and where you are. This means, the official Play app will regular wake your phone up to send status reports back home.
Fun fact about wireless networks: antennas require a lot of power. The higher the frequency (the faster the transmission), the bigger the drain. If all you want from Play is apps, then leaving the official client app enabled just results in your phone having to be charged more often.