Selective attacks on infected planets would leave uninfected worlds intact. The objection is that every Flood spore would have to be eliminated. Firing the Halo Array instead means destroying sentient life throughout the galaxy and starting again. Whether that price makes the broader attack preferable remains unresolved.
Notable Point · 2:21:00-2:21:24
Flood spores complicate a limited strike
A plan to destroy only infected worlds faces the requirement to eliminate every surviving spore.