WHM is the tried and tested healer. Good variety of heals for various situations, generally has an answer to everything. The main area it's weak at is mitigation.

Meanwhile, the constant changes to AST merely highlight how much of a mess the Job has been as a whole. WHM's lack of adjustments during 3.x is a highlight of how solid the Job is, while the current AST buff is like SE desperately saying to players "please add AST to your parties now, we even made the Card you only cared about even better too!"