Or you could finish the fly as you normally do and use the hackle guard as a "hackle press" to press the hackles into place. Kind of like a comb for hackles. Well, that's what you tell the kids anyways.
The other option is to use the guards to hold back hackles while applying head cement. As you get older your hands get a little more shaky and occasionally I paint head cement ont the hackles. Using a hackle gurd as a mask for head cement would probably be useful.