I like to see when other players burn their cards but for me I wouldn't burn them, because like you said we never know what they might be worth in the future.
We are already halfway thru to the total supply of Reward cards (around 40 - 50% already has been distributed). We will see the Dark Energy Chrystal coming up next Month with the feature to melt cards into DEC currency.
So, I probably wait for that feature to make a decision about the bunch of reward cards, because right now they are the most cheapest cards that you will can turn into DEC.
And some of the reward cards are really great for gameplay! The Wood Nymph for example is a great card I would never sell her for 0.03 cents or the Mushroom Seer (rare card) also 0.03 cents lol.