Is "al-Qaeda-linked" a useful designation?

Joshua Keating suggests that because, for example, ISIS has "directly disobeyed" Zawahiri and clashed with Jabhat al-Nusra, it may not be useful to describe the former (or the latter) as 'al-Qaeda-linked.' He writes:

The 2001 Authorization for the Use of Military Force, which is still being used to justify U.S. counterterrorist operations in places like Somalia, pertains to the groups that “planned, authorized, committed, or aided the terrorist attacks that occurred on September 11, 2001.” Dangerous as ISIS may be for its region, it seems like a bit of a stretch to describe its goals as in concert with those of al-Qaida central, circa 2001. As Osama Bin Laden’s successor, Zawahiri, appears less and less in control of the actions of groups like ISIS, al-Shabaab, al-Qaida in the Arabian Peninsula, and others that routinely fall under the “al-Qaida” umbrella in media accounts, it seems like it may be time to narrow our definitions a bit.

I understand the point but I think there may be some reason to treat groups as linked if they share a common ideology, even if their specific aims differ. But it's true that the ordinary U.S. consumer of news, hearing the phrase "al-Qaeda-linked," probably has no clear idea what that means. And there's another point: 'al-Qaeda central' itself, c.2014, may not share the goals of 'al-Qaeda central' c.2001. What are Zawahiri's current goals, other than to survive uncaught and to keep issuing videos?