The thing about multiple decks is that nowadays ROI probability is not as high as before. Yet new players that join are unaware that the rewards scheme was swayed much better in the past. It has always been like this from the start, early adopters earned the most when they started versus players who started a year later and so forth.
I do think in the long run this helps Splinterlands increase value of its cards with the current rewards system. I noticed many cards that were half of what they are now just about 3-4 months ago. Demand on the cards appear to me increasing.

On the note of selling a secondary deck will definitely help spread the cards to more players and at current prices cards seem to be closer to all time highs, which is an ideal way of trading. Buy low sell high.