In particular, if it turns out to reduce infection rate, taking it prophylactically will save the lives of everyone who would otherwise die for lack of a ventilator, or who would die even with one. If it does not turn out to reduce infection rate, no harm done.

If it turns out to shorten the illness, everyone sick, and also everyone taking it prophylactically who is actually infected but asymptomatic, will reduce their contagion rate by joining the ranks of the non-contagious sooner, reducing everyone's risk. If it does not turn out to shorten the illness, no harm done.

Chloroquine is well-known. If it caused an unacceptable rate of liver failure we would already know.

I see suggestions that it takes a very high doses to work. But that might be so only for the already severe infections they are using it on. It should be easy enough to discover whether low doses work prophylactically, and safe enough to try them while waiting on results.