I do not own Kingdom Hearts or any characters thereof, and will not be making any money off of them.
They call it laziness - he calls it practicality. Why run when you can walk? After all, they're Nobodies, so it isn't like they don't have time to spare. And Demyx knows that there's only a certain amount of energy in the world - why waste it? There are more important things in life (or un-life) then Xemnas and his stupid missions. Demyx is the embodiment of water, after all. He knows about glaciers - things that move slowly, oh-so-slowly, at least to the rest of the world. But anyone who has ever seen a valley has seen the sheer power of things, in their slow but steady progress.
He knows that the old members turn their noses up at him, clucking at him sitting in the big room with the couches and fiddling with his sitar. "The laziness of the neophytes," they mumble, and use him as the perfect example of all the things wrong with the new Nobodies. He ignores them, of course. What do a bunch of crusty old Elders know about anything, anyway? It's easier, safer to stay here in the Castle, and at the end of the day, what is more important? A high mission count, or the fact that you're actually alive (inasmuch as a Nobody can be alive). How will he appreciate having his heart returned to him, if he's already dead, or whatever it is that happens to Nobodies when they stop existing?
His water clones are a huge help. They help him with everything. They go into the fray for him, and all he has to do is sit back and direct them, more or less. He plays his sitar and they follow his instructions. Between the water clones and his Dancers, Demyx scarcely has to lift a finger. They do everything for him, from his fetch and carrying to his fighting. He even uses them for sex, because really, why put in the effort of getting someone into bed with you, when all it entails is making them happy, and what, exactly is the point of that? The whole point of sex is enjoyment, right, and it isn't like a person (or un-person or whatever the fuck Demyx should refer to himself as) can get anything from that, anything really pleasurable.
So if Demyx wants to get off, as he does want to do now and again, he summons one of his Dancers, or creates a clone. Much more satisfying, not having to go through all the damned effort and energy, just so that someone would come back to his bed, just so that he'll have something to shove into on a night when he's bored and horny (which must not be emotions because all of them feel it, even sweet, chaste little Xion). So he'll bend a clone over and fuck it senseless (if it had senses) and then dissolve the thing into a puddle of water, reabsorb it, into that big empty space where he keeps all of his power, and roll over, back to sleep. After all, why should he put any effort into anything? It isn't as if any of it matters. He's only an empty shell, and sometimes it seems as if he is the only one who remembers this, out of all of the other Nobodies.