Generally I advise against having both a setter and validator for an attribute (I think I commented on this in my Attribute talk at last year's YUIConf) because:

A) setters can perform the role of validator by returning `Y.Attribute.INVALID_VALUE`, and
B) customizers will likely have to override both setter and validator if they want to change the attribute's behavior

That said, I think this may be an acceptable case if `_setNewValue` were replaced by `_nearestValue` and `_nearestValue` did the rounding seen currently in `_setNewValue` and `_verifyValue`. That change would look like [this](https://gist.github.com/lsmith/49f17181530e6629534f)

Otherwise, I'd put the `isNumber` check in `_setNewValue` and return `INVALID_VALUE` from there rather than add a validator.