The question, I think, is a false choice. In a networked organization such as al-Qaeda, working to maneuver around slowly changing hierarchies with tremendous electronic surveillance abilities, weak ties are at a premium. AQ doesn't need to recruit members and have them move up and mature in the organization in order for them to become useful. They don't necessarily even have to meet them (although meeting could help with indoctrination). Really, they just need to provide a framework or story in which such actors see an affinity between their own deeply felt plights and AQ. Such stories can be disposable in the same sense that suicide bombers are disposable.
In fact, suicide bombers tend to be recruited from the peripheries rather than the centers of social networks.
That fact means that a given loner who wants to play-act jihad, putting himself or herself in the position of hero in a story she or he writes, can find others (AQ) willing to reinforce that story in exchange for credit. AQ provides a story in search of actors. In taking on the role of actor, the loner achieves heroism and affiliation. Think of it as a role-playing game in which you become the hero - as long as you accept your destiny and the constraints that go along with it, the role you need to play.
The affiliation is largely virtual, though, making it very difficult to determine which individuals might become AQ agents until after their attacks. Pursue them too aggressively and you get a lot of false positives. You also get perpetual uncertainty: Who could choose to affiliate with AQ? Where could AQ attack next? And that uncertainty translates into elevated hierarchical responses, increasing the sclerosis in air travel regulation and other infrastructure.
The good news, as Andrew Samwick points out, is that apparently such loners are not easy to recruit. We've had very few attacks, even on airliners, and no one has blown themselves up in Starbucks or under bridges. Possible factors: (a) AQ is not yet effective at detecting and recruiting potential jihadists; (b) the mix of social isolation and fervent Islamism is fairly rare here in the US; (c) political-religious grievances are not as raw here in the US as in, say, Palestine.