Doesn't matter if they have multiple styles of healing to choose from if they aren't going to force that play style onto a healer and make said healer either unable to or not as efficient as another healer at a different playstyle.

WHM and AST being "pure healers" mean they're burst/regen, burst being what you describe as "pure healing". They're both instead of one or the other. I would not mind if WHM became a burst healer and lost its regen and AST became a regen healer and loses its burst. At least then I wouldn't be able to complain that the two were the same and also I could choose which play style I would prefer because they'd both play differently.