For two very important reasons.  

#1 if people are already immune to the virus they can be front line workers, particularly useful for caring for our elderly and other vulnerable populations.  They would also be incredibly useful in healthcare settings.

#2 It is the only way to figure out what the "denominator" is.  How else do we know how far along the outbreak is, and know for a fact just how severe it is?

I am quite disappointed that nobody in a position of power is talking about this incredibly important problem.