The soft starter is actually quite a simple device and in many applications, you must use some form of reduced voltage starter to reduce the starting current of the induction motor. If you are compelled to use a reduced voltage starter to reduce electrical disturbances, or to reduce torque transients, then that is a good reason to use a soft starter which complies with these requirements and is stepless and easy to set up to match the driven load.
There are applications where reducing the voltage, will result in a reduction in iron loss, but this will also result in an increase in the copper loss associated with the load component of the current. If the load current is higher than the magnetizing current, the energy saving may be negative rather than positive.
In the case of a motor that is 95% efficient at full load, the iron loss component will probably be in the order of 2% of the motor rating. At best, you may save 50% of this under virtually open shaft conditions.
As the load is increased, the load component of the current increases and the potential reduction in iron losses reduces. If you have to use a reduced voltage starter, then the soft starter has the lowest losses and no steps. - definite advantages. If Full voltage starting is acceptable, then I agree, why use a soft starter, or a VFD unless the load speed requirements are variable, i.e. variable flow etc.
If the measurements purely on current as I have seen many sales people do, then you will see an improvement in the current consumption, even on a fixed speed application compared to a soft starter on the same application, but if you base the measurements on KW, (what the customer pays for) then at a fixed speed equal to line speed, you will see a 2 - 3% degradation. If you look at true power factor, the VFD is usually worse that the motor at full load and full speed except for special motors, low speed motors and submersible motors. The displacement power factor is improved, but the distortion power factor is low, 0.7 - 0.92 If the customer is paying on true power factor, (KW/KVA) he can pay a higher penalty, not a lower penalty.