I've owned iOS devices since 2007 (starting with the original iPod touch) which have connected to a variety of routers, and I've never witnessed regular occurrence of your problem.
As a matter of fact, over the past couple of years, my iOS devices have very reliably kept on the WiFi network in order to do iCloud backups (typically in the middle of the night).
My iDevices have included the original iPod touch, iPod touch 3G, iPod touch 4G, original iPad, iPad 2, iPhone 4s, iPad mini Retina, and iPhone 5s. The latter two are currently in my possession. For the devices that have been capable of cellular data connectivity, I have always turned off cellular data at home. The cellular data reception at my place is bad, so I don't really need my devices to attempt to access the cellular data network until I step out the front door.
I used Buffalo routers this entire time up until about a month ago, when I switched to an Apple AirPort Extreme base station.
Since you neglect to mention anything about your router and the environment in which you are using these devices, the only course of action one can recommend here is for you to take your iPad to the Genius Bar and have them run a diagnostic on it.
Good luck.