Dont get me started on gearing, there is no such thing as a granny ring, it is a saying that certain riders would have you belive you are not up to there standards, even though my bike is only a 7sp rear, it is a triple, but the range is not that much different to most compact double, mine is usually a little lower but doesn't usually have the higher gearing. I could have dropped to the 30th front, but it wasnt needed, it is a 10%+ hill at points, but not long it hurt because I had only done a mile loop.
My lowest gear as the set up is at the mo is 30x25 =31.56" a compact on on 34x28 = 31.94" so my 30th ring is hardly a granny ring.
Where as my 52x13 is only 105.2" compared with a compact 50x12 of 109.58", but my gears on the rear are closer and not such a big change on the front. I ride a lot on the 52 ring these days, but sometimes because of gear ratios I run on the 40th and if need if climbing a steady climb I will drop onto the 30th nd be in the middle on the rear giving me choices where I might want to go with the gearing if I change up on the front it is only two gears on the rear, some view that a compact double is a better option, but I personally disagree, but it purely my own view point.
But to view the inner ring as a granny ring I think is wrong. I did once ask somebody what they viewed as a granny ring and they said about 22"
which on your 28 would be 34 on the rear providing you are on 700 wheels.
And yes when I started I did use gearing that low.
It was also a bit of a test run to see how the gears work together, as I had a 28/24 opposed to the 25/23 I have at the moment, these being the two lowest gears on the rear.