I quarantine for more reasons then to get rid of ick. I have a big tank with a couple of very active, aggressive fish and everything I get has travelled at least 6hrs in a bag (usually 24hrs). If I were to dump a fish directly into the display I'd probably never see it again.
My quarantine tank has subdued lighting and is in a room that only I go into. The fish has a chance to settle, check out his surroundings and to gain some much needed fighting weight before being tossed into the lions. I also get a chance to look at my purchase and ooh and ahh over it, it could still disappear in the display never to be seen again but at least I've had 6 weeks of quality time with it before it disappears
My achilles, the regal angel and the copperband all went through quarantine and I believe they are all the better for it