It's tricky to restore NAND using IOS bootmii at the moment. The problem is that Wii IOS has to running and that means the NAND can't replace itself when it's running. Someone reprted it was possible but I'm not so sure.
Preloader will give you some protection, it's not bootmii perfect but it's something. A NAND backup, no matter if IOS or boot2 is better than no backup. It only takes a few minutes andf with advances in Wii hacking could become more useful at a later date.