Another philosophy I've always had: always writing for one system means you end up making platform-specific assumptions, even in higher-level languages. For example, concatenating file paths with / by doing "{}/{}/{}".format(var1,var2,var3) rather than using os.path.join(var1,var2,var3) and letting the system handle it.

Developing on one platform and deploying on another means you find more little incompatibilities, and can expose a lot of bugs that failed silently or did the wrong thing on one OS but broke visibly on another OS.