One cannot compare a Macbook Air to an iPad in any regard,
Of course you can compare an iPad and a MacBook Air as purchasing options. As you said, people do it all of the time on this forum. They are different devices, but folks often think of them as alternatives when looking for a second computing device.
The key point is that it is a second device, so maybe they have all of the heavy lifting covered by their first device (say an iMac), but they just want something portable to share with a spouse for routine stuff (email, web, photos, reading, calendar, great battery life etc..). An Air could be overkill, or they might really value portability, or battery life might trump other features, so the consumer pick the iPad as a second device. Or, maybe the consumer want something with more content creation capability, so they buy the Air. Either way the consumer is comparing the two devices and the price and selecting the one that best meets their needs for the cost....so yes, you can compare an iPad to an Air when making a decision about buying a second device. For my needs, the iPad was the more cost effective solution....but that might not be the case for everyone.