MonadIO remains Haskell 98. This means that it has a chance of actually getting into core libraries in a standardizable form and eventually making it into the report.
The problem is there is no full description of MultiParamTypeClasses + FunctionalDependencies that is compatible with all the implementations out there to bless and readily standardize. GHC and Hugs (and older GHCs) disagree on a number of corner cases of how this works.
Worse, the corner cases where they disagree come up because of how GHC desugars things into type families, so describing current GHC behavior really would require dragging into the report pretty much all of OutsideIn(X), which is an order of magnitude more complicated than the entire report today.
So, yes, if we ever want to be able to see the bulk of the combinators in base lifted generalized and have that fact become enshrined in a standard then there is a very real reason to favor MonadIO.
-Edward