I am impartial to being able to transmog weapons to look like legendaries, but If I had to choose a side it would be to let them be moggable. The idea to me that everyone would be running around with transmogged legendaries is unreasonable.
Many of the people I play with prefer to be unique little snowflakes and want their transmog set to stand out because it is different, or rarely seen, or funny.
If legendaries were made moggable, sure for the first few weeks afterwards I am sure we would see people mogging them, for the sake of mogging them. But after standing in the AH and seeing 10 other toons with Thunderfury, or running a LFR and bumping into 6 other batman winged rogues, the "allure" of transmogging an orange would go down.
Not knowing how legendaries are going to work in Mist's I will use the previous model of only legendary weapons. I have had my DWT since early February and am really sick of it. The sets I want to use either don't work with it, or are rather difficult to obtain. What I would like is the ability to transmog a legendary to look like another weapon.
This may be even more of an issue if you make your chest piece legendary, but then can never transmog it so every single sete you make as to incorporate the legendary chest into it.
TL;DR Make it possible to transmog into and out of legendaries.