I believe the real reason WoW did this is that their database didn't really track a lot of holiday gear in any way other than making it white. Some was soulbound - but not all, or they could have just said "soulbound white gear is off limits". Some of this was items like very ld fire festival gear that looked a bit like tank gear while being useable by any class. White gear also included a lot of 'non-combat clothing. You can still buy an assortment of wedding dresses and Korean traditional gowns in Stormwind that are white gear.
- it was just a way to cut down on the work of going through old gear and putting everything into either column A or B.
I'd say gear of the same type, and wearable by your class - as a sensible limit.