Sact is updated much faster than the RPM (in fact, immediately) so if one does not want to risk to start cutting before the RPM is up then one must add dwell delay which is constant. It is true that most VFDs only give information about the frequency, but that frequency at start, when the M3 or M4 command is given, is much more accurate than the Sact, since Sact is immediate, while the RPM information from the VFD is not. It is based on the frequency response of the VFD and is a response based on what the VFD is feeding the spindle motor with, not just a mirrored back message about the frequency command from the software controlling the VFD. I know that at least my VFD, when it says the RPM is at a certain value then the RPM is REALLY very near, and it is accurate enough, milling can be started without further delay, no need for dwell. Additionally there is also a digital flag saying "spindle speed reached" and that flag indicates that the speed is within a few % (parameter defines the %) of the commanded speed. Sact is not the actual spindle speed, it is a set speed, one which we want the VFD to use.
So, since the spindle speed information given by the VFD is more accurate than Sact (during start and stop) in my opinion it is NOT useless to read it, but it depends what you do with it and how the VFD you have behaves.
Normally if you have a VFD which is a vector controlled (with or without sensors) then you get additional advantage. A sensorless vector controlled VFD monitors the motor coil currents, voltages and frequencies, as well as internal temperatures so it isn't actually completely sensorless, and knows when the spindle needs more current because of harder cut or whatever, and in that case it will increase the RPM, so the RPM will not drop. What a sensorless vector controlled VFD does not know is the exact angle of the spindle motor and the measured RPM.
If the cutting load is a problem then the VFD (spindle) is underspecified or configured wrong, or used under wrong conditions (depth of cut, feed rate, RPM, type of tool and so on). Yes, motors cutting air and motors cutting in material are not the same, but the dropping of RPM should not be important and should not present an issue because if it does then something is wrong.
Monitoring of VFD and spindle health is not done through RPM monitoring but through temperature and current monitoring. Those are the parameters to read if monitoring health is the goal.