Per discussions I've been having with developers and accessibility practitioners, as well as parallel discussions I know others have had on the same topic, there seems to be a general question of how useful the tooltip role is when implementing custom tool tips on the web.

Currently the prose for tooltip notes the element(s) it is used on should be referenced by the owning element witharia-describedby. As this attribute is what conveys the tool tip's contents to users when the owning element is focused, it's not clear what the purpose of the tooltip role is meant to be, since aria-describedby would communicate this information regardless of the existence of the tooltip role.

Regarding to how the ARIA specification could clarify the purpose of tooltip, it seems to me there are a few options here from indicating that some user agents may not do anything special with the role, to more clearly indicating how AT could better expose it. I'm sure others have some ideas here though, and I'd be very happy to hear them.