I'm just not sure D'Angelo actually does help the Flyers fill a need, or is really good. He thrived in Carolina paired with a top defensive defenseman. The Flyers don't have one of those (and have also overpaid to retain a poor one). Also, Carolina has a real whiff of being the Tampa Rays of hockey - if they decide to move on from someone it's not just because they're cheap.
Yes, he'll take some of Ellis's minutes (and cap space) but he's also not Ellis (unfortunately neither is Ellis anymore).
So, in total over the past 12 months of offseason trades, Fletcher traded a first-round pick (14th overall), three second-round picks, a third-round pick, and a fourth-round pick, for three right-handed defensemen: Ryan Ellis, Rasmus Ristolainen, and Tony DeAngelo. Ellis ($6.25 M), Ristolainen ($5.1 M), and DeAngelo ($5 M) combine to make $16.35 million, or 19.9% of the salary cap.