Use case which come to my mind is.
1. Display controller might need to flip an image 180 degrees. Here memory reading pattern is reversed.
2. Where ever there is LIFO (Last In Fist Out) implementations.
I'd guess these sorts of functions were not seen as commonly required, so were not considered.
The AMBA specifications tend to be developed from a processor's perspective, where the core logic will be stepping UP through instruction space. So incrementing bursts for sequential stepping, and wrapping bursts for cache line fills.
A display controller needing to flip an image would obviously be implemented more efficiently if you had the option of a decrementing address burst, but this wouldn't be considered a common enough application to merit a new burst type in the protocol, and so this operation would need to be implemented using either lots of shorter incrementing bursts to write out buffered data being reversed, or else if the requirement is that the entire image is written out in reverse order, you would need to use lots of length 1 transfers.