First there are surge protectors and surge PROCTERS the Hw-50C is the latter.
I've read of transfer switches and other devices with built in "Surge protection" but what I see when I look at them is spike supression. A Spike is a brief high voltage.. They do nothing for you if the voltage at the oullet is too low and if you plug into a 240 volt 30 amp outlet (Don't laugh and do not say it can't be done.. Happens all the time. often because somebody mis-wired a TT-30 or failed to read the "Max 125 volts" printed clearly upon it)
Well the MOV's in those things go off like July-4. Firecrackers.
Now the HW-50C will shut you down if the voltage goes low
It will shut you down if the voltage goes high
And it will protect you from Spikes.. Without firecracker effect
So yes. you need one.
I would put it in the line on the shore side of the ATS.. I don't worry so much about my generator. (Onan's monitor their output and shut down if it goes crazy)