Totally agree. I haven't spent any DEC since SPS started and it's definitely increased the SPS that I collect... but... it also means I'm not upgrading any of my cards at the moment which is a real shame.
I put an example in my own Season Report, but I could spend 1000 DEC on a card that would only increase my power by 40 DEC (and therefore my airdrop points by 40)... but I'd have a stronger card that might help me win more matches... so yeah, I don't know what the balance is.