@wkempf: In principle I agree with you. In practice however, it would be disastrous if IE came out with a completely incompatible specification, no matter what the reasoning. Projects like CoffeeScript and Script# do not have the time or resources to put into multiple source map generators. Then web developers will be faced with the problem that their tool of choice only generates source maps for chrome, but not for firefox or IE, or the other way around. Or, compiler writers will simply forgo source map compilation altogether because nobody can agree on a specification. The best case scenario is that they are similar enough that one could write a tool to translate a source map from one format to another, but that could result in a loss of information.
So, it would be better for all stakeholders involved if IE implemented the google standard, or else worked with both google and the Mozilla team to improve the standard together before implementing it, possibly even formalizing it into a web standards body proposal together. In this ecosystem, there is only room for one specification, so it's better to work together on it and not introduce a competing format just for the sake of inventing something in-house.