If each encounter was the one to one fight, I'd agree, but based on the information, the Sisters were initially decanted with zero knowledge, and level 2 powers, as well as the knowledge on how to use a variety of weapons. Nothing at all. Accelerator was essentially fighting babies with guns for the initial fights, and likely a few attempts to take him down by coordinated mass combat. To the first Sisters, it was a mysterious death, if they attacked him, they died. If they were touched by him, they died.While this is true - seriously, over ten thousand deaths and they're still following the same general idea?
I suppose it's a way to show just how little the value of a Sister's life was, even in their minds, if they were willing to go through with it.
Apparently it doesn't work properly for shields. She could do point defense shootdowns, but she's rarely ever made a barrier, and it's also notable that using her power for defense severely damages her from the burn.Technically, she can use her powers for defense, too, by creating shields made out of that power which annihilates anything it comes into contact with. Kinda like the Narutoverse Truth Seeking Ball technique.
Oh, it can get far more defensive and versatile than that, and he's used it for said purposes as well. Movement? Even leaving aside air control and flight, we see him use it to cover large distances in an instant. Alteration of states of matter? Done (Gas to plasma). Rearranging the landscape, for whatever reason? He's done that too, and with far more precision than mere destruction.
It's a highly versatile power. And it's not like 'destruction' is how it grows, either - the gains in the Touma fight show it pretty clearly.
Destruction is how he sees it, and how the Kihara in charge developed him. All those things aside from the high speed charge attack came about after he saved Last Order, and realized that he could use his power for more than just killing and shrugging off the world. The factor there is perception, and with an esper power, perception counts for a great deal.