This. It’s becasue there are rogue specific attacks that require daggers. Those attacks use animations that were designed with daggers in mind, they are some stabs and stuff. It would look odd to stab people with an axe, fist or mace. Swords could be ok but this would exclude them from being able to mog into axes or maces and fists.
No matter what spec he is they can only mog dagger to dagger. I linked the guide lines on transmog in this thread please dont give the wrong infomation.
You can wield a axe off hand and MOG it like I have my dagger in main and sword off hand but. Dagger can only be mogged into another dagger that should be obvious