Hi Judd, this sounds very similar to the POE experience. I think someone mentioned in the LE Devblog about POE's party play which is that POE appears to be a single player game with multiplayer as an afterthought. D3 and POE are online only ARPGs so even single players are compelled to partipate in the online community somewhat. But with LE offering offline play, it appears there are little incentives for playing online? (PS: I understand leaderboards and such are mentioned, but other key attractions for online such as multiplay and trading appears limited, I am not sure the former is sufficient to incentivise players to go online?)
There are lots of reasons to play online! You have access to races, leaderboards, PvP, group play, trading, chatting, safe storage of your character server-side, the ability to gain XP/items a bit faster if organized, some class synergies (think paladin with heals or warlocks with debuffs). The team expects to see over 80% of players play online.