While the "greater good" argument has sense, and was even used in the Archie continuity as memory serves, a problem arises in the simple fact he either has to leave the Emerald unguarded or take it with him, thus causing the Island to fall. I'm confident Knuckles is well aware of the fact the Master Emerald needs protection for a reason: it has such immense power and capabilities that it can't be allowed to fall into the wrong hands. So it's reasonable to assume he'd have some sort of backup plan to keep it secure whenever he ventures down below... the problem is Sonic Team's never elaborated. Based on the dialogue in Sonic Adventure 2, it sounded like Eggman was expecting a Chaos Emerald, but found the Master Emerald instead. He more or less seemed to just shrug about the situation and figured he may as well take it while he was there. So it was spur of the moment; as such, we can conclude Eggman has indeed given up on stealing the gem based on the main titles. Which I find incredibly surprising since Knuckles is down on Earth half the time anyway so it's strange a man of his genius hasn't dispatched some sort of small task force to "borrow" it while Red is busy elsewhere. Then again Eggman does forget the minor details sometimes... Hmm, somewhat, but I meant a more comprehensive trap system. Knuckles' system was horribly flawed since he had to be present to trigger each one; my concept of effective traps would allow him to leave the island, whereas whoever stepped within so much as a hundred yards of the Master Emerald would find some bad fate or another awaiting them. His trap system in Sonic 3 was very crude and ineffective as it necessitated he be there to activate it manually. I think it's safe to say he can most definitely carry it around with him (he has a reputation as the strongest, even), though the actual size shifting likely is just presentation. It looks less awkward if he pulls out a medium-sized gem versus one that's gigantic. His (apparently) psychic link with the Emerald definitely has its uses, as he'll know if/when the Emerald is in danger (as he did in Sonic 3&K) from afar. My main problem is that even if he carries the Emerald with him to keep it safe, logically that means Angel Island falls into the ocean. From what we've established, this is an undesirable outcome, so it leaves us with a bit of a problem: Knuckles can't simultaneously have the emerald on his person and keep Angel Island safe in the heavens. That's where a problem arises though: how can he adequately protect it if he's halfway around the globe? The Emerald is safe of course, but if he's not on the island there's nothing to keep it safe. Unless he has some teleportation ability or has set up automated defenses/assistance on the island, there's nothing there. Seems far too utilitarian for him to just write the island off on the basis of reclaiming it later, I'd think.