MGS2 would've been a lot different if there weren't loading screen dividers. It would've made the game a lot more challenging if guards as an entire unit moved around the tanker or Big Shell in different shifts, or if they can chase you around a connected areas and kill you without a loading screen checkpoint being a safety net if you die. Save games being the only checkpoints would've also upped tension and challenge.
The loading screens themselves aren't obtrusive, but each transition being its own checkpoint lowers tension a lot unless you're doing a Big Boss rank run. MGS2 has great gameplay, incredibly puzzle like in nature, but a lot of that is in the scenario based VR missions and Snake Tales. Harder difficulties in the main game lower resources, which make things a lot more interesting.
I think everything being connected without loading screens was an idea during MGS2's production, but the team couldn't get around that hurdle.