I think the main reason was to let people show off their retainers. Nothing to do with sales. I know some people who have iLvl 110 retainers since they got all their gear handmedowns. They are like, STUPID PROUD of their overdressed retainers, and would love for someone else to be able to see them.

Me, personally, all my retainers are dressed for business and too busy harvesting and mining to bother with showing off.