Some 6-mans like Thadur, HV, SG for completing the challenges... as it should be.
Most 12-mans award them. Join up with a group running Barad Guldur Tier 2. You'll get 32 seals for the whole run if you complete all the challenges, plus additional rewards.
The instances that do not drop seals have (to some extent) received an increase in the other rewards/currency. Just about every instance you have a pretty good chance of receiving a "teal" jewelry, armor, etc... which makes getting those things less of a grind.
The Big Battles are quite fun... and a good addition to our entire repertoire of available instances... if you are willing to take the time to learn how to run them properly and put forth a little effort. FYI, just standing there afk for 20 minutes does NOT just let you win, let alone give you a chance at the nice teal/gold jewelry that you can get from them.