But why only the common unix commands? I can't think of a level of caution where filtering 'echo' is appropriate but, filtering a more obscure command, perhaps

But why only the common unix commands? I can't think of a level of caution where filtering 'echo' is appropriate but, filtering a more obscure command, perhaps 'dc', is not. An excess of caution would be to put the process in its own chroot'ed jail, with only the bare minimum of tools for needed functionality. And '/bin/echo' is not one of those tools.