He was a great photographer and (by most accounts) a nice person too. But there is a distinction between AA and some of his followers, just as there is a distinction between Christ and some Christians (the sort who burned witches, for example).
Everyone can learn an immense amount from AA, but it is true that he was increasingly trapped in a single style of photography, largely to please his fans. Some of his early advertising work was superb -- a bar interior springs to mind -- and I've even seen a little of his reportage. Again, brilliant.
Today, many of his followers are addicted to the same subject that made him famous -- faux wilderness -- and imagine that the Zone System is the origin of sensitometry rather than a restatement of its basic principles, which were well established long before AA. Distressingly many are also convinced that the Zone System is what made him great, and that if they can attain the same technical mastery, they'll be as good too. The truth is that he was simply a brilliant photographer, even before he invented the ZS, but the ZS is easier to discuss.
In other words, if someone appears to put down AA, it's not necessarily the man that irritates them: it's some of his followers. The same applies to faux-wilderness pictures.