Noctis sums it up pretty well:
Specific to shoulder tackle and mercy stroke, having your gap closer on CD when you actually need it turns into a DPS loss, and using mercy stroke on an add when you need it on something that is burst dependent is a strategic loss. With mercy stroke having a very long CD, it is more important to use it where it needs to be used, unlike BRD's misery's end.
In the end, it is simply unwise to automate the usage of abilities that have more strategic value than pure damage.