Of course this isn't to say that a widely recognisable sample can't be effective - look at Tim Roth's dialogue from Made in Britain in that one Genocide Organ track, and I've even been guilty of sampling from Scum (but I guess I look at my own project as more of a hobby than serious art).
Even if something is widely available, it could be used in an interesting way, given the context. Koufar sampling from The Wire is a good example of this- cements the idea of the project as modern, tough, and real.