There are other important security related fixes: About the security content of iOS 8.4 - Apple Support
Here are 3 of those fixes:
CoreGraphics
Impact: Opening a maliciously crafted PDF file may lead to an unexpected application termination or arbitrary code execution
coreTLS
Impact: An attacker with a privileged network position may intercept SSL/TLS connections
Safari
Impact: Visiting a maliciously crafted website may compromise user information on the filesystem
If you ignore iOS 8.4 update, you'll still be able to download/install newer iOS if your device is supported. You might want to leave WiFi switched OFF while charging your iPad. If you leave it ON, your iPad will start downloading iOS 8.4 whether you like it or not. Installation is NOT automatic. Once the update is fully downloaded, you'll be prompted to install. You may choose to ignore it if you like but iOS 8.4 update has taken X amount of space on the iPad storage.