What makes Superman Superman? Is it his powers? Lots of other heroes are as, almost as, or more powerful than Superman, so it's not his powers alone that make him who he is.
What makes him Superman is his dedication to truth and justice and "doing the right thing". He is about as "good" as a man can be - a beacon for us all to aspire to. But with the powers he has, that presents Clark with a moral problem...
With the powers he possesses these days (not so much in his early days when his powers were much more limited), there is never a time when he isn't aware of someone, somewhere in trouble. If he's going to be "good" then how does he deal with this situation when being Clark, for example? For every second he's being Clark, working at the Planet, wooing Lois, visiting Ma Kent, he's letting someone, somewhere, die or come to harm. Knowingly, deliberately and seemingly uncaringly.
Kurt Busiek examines this problem in Astro City #1, where the hero, The Samaritan; a clear Superman analogue, shows us a day in his life. The only moment of respite he has is when flying between crises and disasters - and even then he still worries that he can't be 'omnipresent'.
A true 'Superman' would never be 'off' - could never take a moment to just be normal, could never be Clark in other words.
Unless...
Picture the scene. At first Superman does exactly what Samaritan does - never takes a rest, with just those brief seconds flying between crises the only respite he has. While Superman would never get physically tired, mentally and emotionally the strain would bound to show eventually.
And that's where you need other superheroes to step in. To take Superman aside and say "let us take up the slack - take some time off dude".
Then and only then can Superman take a moment out of his busy schedule saving the world to just be Clark for a moment, to do some 'normal' work, to connect to other people on their level, to relax - to just be a man instead of being Superman 24/7 365. Because although he still hears/sees people in trouble, he can ALSO hear/see that other heroes are 'taking care of business'.
Without these other heroes (whoever they may be), this will always be the BIG elephant in the room when it comes to questioning Superman's existence and morals.
Nolan wants 'realism' in his movies? In order for Clark/Superman to be remotely realistic as a "person" (putting aside the fantasy of his powers) and to be able to stand up to critical scrutiny of his morals and behaviour, then other superheroes are an absolute necessity.
To have him in isolation raises far more problems than it solves. To have him in in a world with no other heroes means to question the very ideals he is supposed to represent unless he spends ALL his time as Superman, never being 'off', never being Clark. Or, alternatively, de-powering him to the point where he can't be everywhere and do everything, which is equally undesirable in my opinion. Superman NEEDS the Justice League and the Teen Titans. Superman needs the DCU. You know it makes sense Chris.