>>713710713ABI only really starts mattering for final evolutions. Eventually, you'll have to de-digivolve at least once to get some ABI for the lower tier (not necessarily talking about their actual strength, just how the game judges them), then a bunch more if you want the real fancy ones (demon lords, etc.).
I wouldn't really start with a big de-digivolution orgy until you have enough party capacity to have three digimon at ultimate level and three at the champion level so you can have a strong backup team and don't get locked in the story after de-digivolving your main team.