The Breton knight costume back? If that is what you mean then you merely get 3 different knight costumes that can be worn by any race or levl that can't be dyed.
They offer no extra armour rating other than what you are wearing "underneath"
Costumes don't have armor rating, they're purely cosmetic (you retain the rating of armor worn under costume). They also can't be dyed, which is a shame.
All costumes are only cosmetic, nothing more. No armor, no requirements, no dyeing.
Is it worth to -you- looking like -that-?
For some it may be. Could be fun for a guild for example to have all their members in "uniform" when going to PvP (would also make it a bit more difficult for enemies to pick out the lighter armored ones from the ranks)
But for many more it might not be worth shelling out those crowns...
Make your personal choice!
I bought the Knight and Ordinator pack since console launch, however. I only wore them until armor styles started changing in appearance from the late mid levels and I haven't put them on since. The early crafted/found armor models are displeasing, in my opinion, so wearing a costume at least allowed me to not feel like a thirft store warrior.