Plural, number, choice, etc can be specified in a message format string, but 'measure format' cannot be.
Chrome has strings like '2 days and 11 hours ago', '10 hours and 23 minutes remaining', or '5 minutes 25 seconds left'. Currently, Chrome combines two plural strings to generate strings like that. (the order is assumed to be large-to-small units and it may not be correct for all locales).
If we can specify a measure format in a general message format (as is possible with plural, gender, choice, etc), Chrome can avoid that.
Related to this issue is 'duration format'. 'x <unit(s)> ago' is supported by Duration Format, but 'x <unit> and y <unit2> ago' (let alone combining that with other strings) is not supported. So, Duration format does not help much here.