A) Sorry for calling you a guy
B) I'm annoyed
C) It's simple tactics. While throwing all your cards on the table is fine, I prefer something a bit deeper. Putting everything I could possible think of with this power would be lengthy. And it'd also mean that everyone else would immediately be defensive and would face my character, if they needed, knowing exactly what they were up against, and figuring on whether or not they could legitimately beat me through IC accidental discoveries and such. If no one realizes that my character can only use three useful weapons, then they will expect every weapon he uses to be deadly- if he summons a halberd, they don't know if it will actually cut them in half, or disintegrate. I have bad experiences with putting all cards on table and letting everyone trust everyone. That and I like something a bit more complex.
D) Someone who controls air could suffocate anyone. Someone who controls shadows could kill a man from the inside out, because, believe it or not, there are shadows inside one's body, because there are empty spaces inside one's body. A small dagger made out of that shadow could pierce the heart, or the intestines, and it'd be plenty enough to kill or disable. Likewise, someone who has the ability to control water could probably control the acids in one's stomach, or the liquid that is actually in our bodies. We may not be made out of complete water, but there is water present. Someone who controls earth-like elements would be able to lock anyone in place, and launch a small rock at someone that could completely shatter something. A bullet is a tiny little piece of metal. No matter how hard you throw it, it'll never pierce skill. Take that bullet and give it velocity, and it can make a man disintegrate. Any power can be overpowered. Any power can be legitimate and still be overpowered. These are superpowers- controlling the growth of plants could mean making a grove of highly poisonous plants that kill anyone that comes near, much less tries to hack through it or push through. Or just put them to sleep. Shifting into an animal would, if you did it right, mean an unmatched edge in combat.
The difference is, my power is not directly affecting. I cannot say, "You are dead because I dropped you into an alternate reality where there is nothing." I can only say, "I brought this sword, and am bringing it upon your head." Likewise, I could also say, "I have this alternate version of me, but because their power and actions take up my strength and power, I could not effectively fight with them, because their power and strength are more than mine, and so their using it would mean my collapse."
Hardy hardy har. You kicked a power that's perhaps the most harmless of them all.
I can't make a flood of water. I can't read someone's mind. I can't redirect and aim fire itself. I can't control the bloody shadows. I can't teleport, I can't heal myself, I can't do any of that. The only direct thing my power does is keep me from dying, by importing an alternate. I can form a tool or weapon, or let me wear some clothes. But I can't be nearly as 'dangerous' as any of the others.