In the shared fact dialog to add a role to one person, you are given an opportunity to choose a role from a list of the available roles. When you add a role to multiple people, you are not given an opportunity to choose a role from a list of the available roles. RM simply chooses the first role in the list for you.
Not being able to choose a role from the list of available roles when adding multiple people seems like a bug that needs to be fixed. It does not seem like adding the ability to choose a role from the list of available roles when adding multiple people should really be viewed as a requested feature enhancement. But I would rather see this addressed in RM8 rather than RM7.
RM in general is very weak in supporting any kind of bulk data entry operations. By design, it doesn't support bulk delete of people. Two of the places it does support bulk data entry to a limited extent are MultiCite and adding a role to multiple people. I would have to think about how MultiCite could be enhanced, but it's easy to see several useful ways adding a role to multiple people could be enhanced. Such enhancements could include fixing the problem of not being able to pick a role, adding the ability to share the Principle role, and adding the ability to share a role as if it were a true fact. Combining the last two would give RM for the first time the ability to copy a fact to other individuals in the database. I think RM users would quickly find all sorts of amazing and wonderful ways to use such enhancements.
P.S. Even if the ability to copy facts to other individuals were added, RM still needs to the ability to export roles as facts - in GEDCOM, to FamilySearch, and via TreeShare to ancestry.com.