You are absolutely correct, K panic's won't show up. However, in our environment we measure kernel performance in milliseconds (more precisely - microseconds). Tacking 'silent' to the kernel as a boot option still requires processing and every printk/silent flag evaluation still requires _at least_ two or three extra CPU cycles.
You can accomplish the same by appending "silent" as a boot option. It skips over any printks, but you have the same problem that you do with removing them from the source. Any panics will never show up and you still are stuck with the printfs.