The Marcuses work to change the world, by changing worlds. It's not exactly a "preservation" philosophy. Plus, they are in the business of creating potential for life, so what's the point of worrying about loss of such potential in Genesis detonations? There's no loss!
What's on the planet is controlled circumstances. Without those, science is invalid. If Carol Marcus cooks life out of existing life, it's not the same as cooking life out of lifelessness.
It's a different question what the required circumstances really were. Apparently, the perfect world for Genesis has oxygen but no life, which is probably a pretty rare thing - and might explain why Terrell and his crew had to spend so much time searching, and were getting so tired and careless toward the end. But why the need for such? In the simulation, it seemed an airless body was to be converted.
(Or was the presence of breathable air on Ceti Alpha V something our Reliant
heroes initially missed or misidentified, just like they missed Khan's crew and the eels? Doesn't sound likely. And the two officers readily removed their helmets without expressing surprise at the prevailing conditions...)