I find that interesting. At work, and between my family who is on AT&T at present, these people have maintained their unlimited data plans. I've been with them when the texts have come through, "You have reached 3072 megabytes of data, you data is now being throttled", ect. Hell, they even showed me because they were confused at first.
Even before I switched, I remember AT&T being VERY accurate with texts notating when you hit a certain data point, such as 65%, 90% and over your limit for tiered plans. Actually, I recall this happening in both Boston and NYC, so it's not an isolated incident to a certain "market."
I think maybe what's happening here, is that AT&T only monitors and notifies you ever so often, not instantly with data updates. Thus, you could pass 3Gb at 12:00 PM, and continue to hit 3.7Gb, at which point you would be warned with a text and throttled.
The issue with subscriber count is not indicative with overall value and customer service experience. If it were, then Verizon would be top dog as they trump AT&T in all three of those categories. Again, read the third party reviews on these things, this is not my bias, but the conclusions drawn by major customer satisfaction outlets. It just so happens that my personal experiences match the commentary by these third parties ...
If you are happy on AT&T, then that's perfect, great for you. We all pay enough per month to deserve a good cellular experience. You have good experiences on AT&T, but I do not. We are both entitled to our own opinions. Clearly the OP is not having a great experience either, hence he's jumping ship, even to Sprint of all carriers.