Well yes, but..
1) If it's sitting on 100% then it's not your "prized" piece of gear that you're afraid to lose.
2) Putting it on your retainer involves going to the exact same zone line as getting the certificate could... and talking to an NPC... Or if you're a Twin Adder or Maelstrom member, getting a certificate would actually take longer because you'd have to fly to Grid or LL, as opposed to just entering the Ul'dah Market Wards.
Also, you're saying that putting a gear piece on your retainer would take just as long as trading it in? I think you missed the point of the post you quoted earlier... The problem was that, if you traded in your "prized" piece, you might have horrid luck and never have it replaced. If it's your prized piece, wouldn't you be willing to go through the extra ten seconds of effort to keep it safe? o.0
I'm not sure exactly what problem you forsee. ^^; That you might have to take an extra trip to get seals instead of an NM drop? The post you to which you said "Yeah, this." proposed the certificates, which would take longer than putting it on your retainer...
But you're right, they're just workarounds. But if it's your "prized gear" you should be letting someone else in your party have it to keep, unless everyone has one that they use every day... With the retainer option, you obviously couldn't
use your verdant scepter to get another one, but if you were using it, it wouldn't be at 100% and you could just request repairs.
@Asiaine: In this thread, I asked for a way to get rid of
useless NM drops besides tossing them, and I got exactly what I wanted. ^^; I realize some of you read "We are giving you another way to farm seals!

" but since it's in this thead, I get the feeling that you're missing at least part of point. If I had to get a certificate to convert my item upon drop, it wouldn't solve my issue unless you had the certificate for every piece of U/U gear in the game...