So, this is the one thing I really don't get as a complaint.
The cinematics of D1 are barely worth mentioning in any context. You get an intro cinematic that sets tone and nothing else, and you get a closing cinematic that, well, closes the game. If we're talking about D2, however...
Okay, this is a peeve I've always had with D2 (then again, I've never particularly liked D2, but that's another issue), but while you're right that things are "happening on the screen," so to speak, the cinematics are segregated from what's actually happening in the game itself. Marius's story is largely separate from the player's story, you could cut the cinematics out entirely and miss little context, and you get this weird case where in-game, the player beats Mephisto and goes through the Infernal Gate, after which the Infernal Gate cinematic plays which shows said gate being open and Mephisto in its glory. It feels like something that was meant to happen before the end of Act III, but for whatever reason, is here.
I could get making a comparison to D3 which has the best of both worlds (cinematics between acts with characters that appear in-game, not some rando narrating), but while D2's cinematics are undoubtedly flashier than Lorath's voiceovers, the voiceovers, at least for me, carry much more narrative heft. Lorath is known to the player, Lorath is telling the story that he himself is involved in, and we run the gammete of despair (Acts 1-2), to hope (3-4), to unease (5), to the ending of the game itself. YMMV, but the ending of D4 is excellent, while the ending of D2...isn't. Seeing Marius brutally murdered by Baal certainly fits the grimdark tone of the series, but in terms of character and narrative? It can't even come close to D4 (heck, it doesn't even come close to D3's vanilla ending).
