As proposed in React Future, it would be nice to support a list of style objects in the style property. The objects would be automatically merged (in order, with the last object overriding the previous one).
var boxStyle = {
padding: 10,
background: "hotpink"
};
var bixBoxStyle = {
margin: 10,
width: 300
};
<div style={boxStyle} />
<div style={[boxStyle, bigBoxStyle]} />
<div style={[boxStyle, bigBoxStyle, {background: "navy"}]} />