I think the guideline is a little outdated, from the time when MusicBrainz was more focussed on just music and there were no artist credits or even a difference between recordings and tracks.
I’d say that it’s better to avoid using special purpose artists if more specific information is available. It is a useful special purpose artist if we don’t know who perform the dialogue, but the pure fact that it is a piece of dialogue and not music should be captured somewhere else. In this case, it would be a more interesting discussion whether this should be credited to Jules and Vincent or Jackson and Travolta?