According to their networking presentation at GDC, quite the opposite.
The servers handle matchmaking and high-level event scripting, in the form of "activity hosts." This is nice because, for instance, if there's a problem with a P2P host, a stable source is making sure that the mission-critical details don't get screwed up in the migration (in the GDC talk, they use the example of multiple flags spawning in CTF in Reach if something bad happens with the host).
But the actual moment-to-moment physics and AI and such in an area? Peer to peer. In cases where players are physically close together, this means that Destiny can enjoy very low latency, and importantly, it means that Bungie can get by with only a tiny server base. If the "heavy lifting" was handled server-side, Destiny would have been swamped at launch, just like every other major online game ever.
It's not like there can only ever be one reason behind a decision. The "gameplay reasons we made these choices" all happen to align
strangely well with a need to cut down on bandwidth.
That would only be true if the P2P host had to handle all areas that the party was present in at once. This is the cleverness of Destiny's system; P2P hosts handle individual areas, and the server-side activity host splits and rejoins the P2P instances as needed. In your example, the players in different areas would be communicating with different physics/AI/whatever hosts.
This is why people other people in your party vanish and reappear sometimes when you're moving between areas; you're literally being split off and rejoined into little P2P instances. If the servers were handling physics and AI and such, this wouldn't have to happen.
