Except for a 1/4 wave vertical, why would one need ground radials?

For receive, you really don't need any to "work". But like one said,
adding more will lower the overall angle of radiation and would improve
it for low angle long haul stuff. But how much vs the price of wire
would depend. For receive only , I doubt I would use too many radials.
The payoff is not really worth it, if you don't intend to transmit. Of
course,
if time, $$$$, etc, no object, many radials are better than just a few.
So for some of the harder core, it might be worth the trouble.
MK