The link given above suggests that there's some disagreement among respected sources as to whether a chord has to have 3 notes.

All I can say is that in folk music (not just ours but other countries' music too), the open fifth is often used without the intervening third - melodeons often choose to do this, as do some guitarists. Some singers tend to do it too, when harmonising - although many probably do this without realising what's going on - they avoid using the third of the scale as a harmony to the tonic, and often stick to the fifth.

This gives a spare, slightly raw sound, less cosy and complete than the triad, and the fact that is is unresolved gives the sound an edge that many people like, and which sounds right for some tunes/songs. And yes, a fifth (say D and A) will occur in more than one chord/key, but the context of the melody and the other chords will usually make it clear what's going on harmonically.

Marje