I think inaccessible doors/locked doors without keys is a bit crap when it comes to game design. In design, I believe everything should have a reason and explanation (if requested) on why it is there. I really shouldn't have to think "Oh, I went through the entire game and I still haven't found a key or a way to open this door." so I have to leave the game and look it up to see if there's a way to open it.
I hate that, it breaks immersion. If there's a locked door, I would rather spend hours figuring it out or building my lockpick skill to see how to open it and being rewarded for that. Instead, if there's always the idea of "Oh, well, maybe this is one of those doors that doesn't open..." then that doubt will be in my mind and I'll end up looking it up just to ruin the surprise.
If it's hand placed in the game world, then I think it should be apart of the game. There are visual/usability strategies to make an inaccessible door less compelling to mess with (visually broken doors, creating conventions early on so the player "knows" what door it can't access) or give solid reasons why the player can't get in (saying "There is no key, it's lost or something! Move along and play the game anywhere but here because we say so!" is a cop out in a world where keys are collected and points are spent into lockpicking skills).
That's like having four buttons on a computer console. Three buttons do things but the last one brings up a message "THIS BUTTON DOESN'T WORK." Why? Because the console creator thought the console needed more buttons to make it more believable, so they added things that look like they are important but really aren't at all! Horseshit. If I see a door that doesn't open, I should never have to stop and think "Oh, this is here because the game developers want to insinuate that the area is bigger than it really is" instead of me actually being convinced that it is.
But if it's an average door that the player can safely expect it to work, and is even interactive, then it's just a disappointing tease.
This is like the "game boundary" argument where no answer is really natural feeling. Also, I'm really just being a baby.