Shackles are reliable and strong, and excellent indicators of when a block might need replacing. Deformation or distortion of the shackle is a good warning sign that the block is working at its maximum load capacity and could be due for replacement or service.
On the other hand, soft attachments aren't perfect for every situation. For instance, rope attachments are more susceptible to wear and chafe, and require inspection more often than a shackle.
Soft attachments require smooth radius attachment points. Sharp or rugged edges, like toe rails, can cause wear/weak points to form in the rope. If edges are rough, using a shackle would be more appropriate. If the surface has a smooth radius, then a lighter soft-attachment would be indicated.
Select soft-attachment whenever greater articulation is required and weight is a consideration.