I thought all the iPads fit.
Since the holder allows a bit over 170mm, every 9.7 inch iPad from the Air 1 and onward
should fit without a case. Also of course the iPad
Mini 2/3/4. Some people just have more of a comfort level either using their tablet
with the case on or with simply having more 'clicks' available to lock their tablet in place so they prefer the extender. The iPad 4 is the oldest 9.7 inch iPad with a lightning connector, but it is 186mm and won't fit without an extender. It is also
much heavier than the later 9.7 inch models, so it may not be very comfortable for some to use for long periods of time.
My personal experience is with an iPad
Mini 2 and with an
Air 2. I used the
Mini 2 for only a short time due to overheating, so I've exclusively used an
Air 2 for about 20 months now. The
Air 2 fits without a case, though my holder allows for only a single click to lock it in place. Since the
Air 2 weighs only 4 ounces more than my old
Mini 2, it still feels very solid and not too heavy when mounted on the RC with a metal arm. I never even think about it falling out. Again, that's just my personal experience. As I said, some just feel more comfortable with a case on or with more clicks to lock it in place.
Sorry for being long winded!
Cheers!