Why is 2.0 Dengar's ability charge based, when they said that you can only perform 1 extra attack per round. Is there something I'm missing?
Dengar 2.0
Tracking reasons. They said everything that can only trigger once but doesnt trigger ONLY at a single point each turn has a charge purely for tracking
Dengar can trigger potentially up to 8 times, so it needs a tracker so you dont "forget" he did or didnt yet
Makes sense I guess 
Note, he has a regenerating charge.
It seems like most if not all abilities that were once per round or discard, are now based on charges, regular or Force, so that they are trackable and nothing has to be discarded so all text remains visible.
Charges will be used going forward for any ability that is essentially "On or Off." In Dengar's case, his ability turns on at the beginning of a round and turns off after you use it. Some abilities will allow you to recover those charges; others will not.