I already had my first emblem from Humbaba, which I'm saving up for when I need to spend it to unlock Animal Kinship 6. However today, I donated enough to earn a second. I can't auction it, and I can only Caress/Contemplate/Consider one emblem per giant per day, is it useful at all?
There are three things you can do with the 'leftover' emblems
You've mentioned CCC, which is actually one of the main reasons why you might consider collecting a second set of emblems. With 11 emblems, you can obtain a fairly large amount of energy, mood and xp per day. Carrying around 11 emblems can be a bit of a pain, but it depends on much you value this source of 'free' energy and mood.
Spend it on your next skill
Emblems can be spent to reduce skill learning time. Each emblem is equivalent to 1000 favor. Primary/Secondary Giant effect will count, so it's best to keep this for AK7 if you want to do this.
Make an icon
An icon is basically an upgraded version of an emblem's CCC features. You can similarly Ruminate/Revere/Reflect for a considerably larger amount of xp, energy and mood everyday. However, icons require tithing of currents, which can get quite expensive with higher levels. An icon requires 11 emblems of the same giant to make. You can find out more at the announcement thread on the forums.
Do note that in addition to these direct uses of emblems, hitting maximum favor with a giant and obtaining its emblem will also increase the daily favor limit, which is equivalent to the amount needed to obtain the next emblem. This allows you to reduce the skill learning time more quickly, which is useful for skills with long learning time, like Mining IV and Better Learning V