As far as genetic modification of oxygen goes, you're right. It's much better to terraform a planet than to dabble in genetic engineering, mostly because modification rate is just so slow (250k people per year per installation isn't really that great). However genetic engineering allows you to settle planets that would otherwise be out of your reach. Most importantly if you invest reasonable amount of RP you can colonize really, really cold planets that would otherwise not be habitable. For me it's also good as I play most of my games with human gravity tolerance set to 70% which means I cannot normally colonize bodies like Luna or the Galilean Moons.