Crusader armour was somewhat decent when it was implemented but crafted armour with skill enhancement attachments is far more effective.
If you like the look, go for it though, and as far as I know it is factional so you wont be able to equip it if neutral or opposing faction.
The appearance system is currently in development with no estimate for when we might see it, the idea is that you equip armour as usual, then have the option to set any other wearable as an appearance.
So for instance, if you want the stats of the Crusader Helmet, but like the look of a Swoop helm, you can wear both and the Swoop Helm will show because it is set as appearance and the stats of the Crusader Helmet will provide protection.