Several people now have hinted that it's all about putting a personality across. Anyone who is inclined to sing is likely to be giving the audience more of themselves, just because a voice and words are literally more human than noises made on an instrument. If you only play tunes and don't say anything interesting in between you wont get far. If you play tunes and you can be entertaining in between, or even while you're playing, or if your playing is so stunningly good it creates real excitement, you can be successful.

In jazz the excitement comes from the fact that the players are improvising so they are 'on the edge' and creative every moment.
With Phil Cunningham and Aly bain it's the crack between the tunes - with Ali Anderson it's the chat and the way he plays - with classical music it's the complexity of the music itself - with Vicki and Johnny it's the goofing around between, the comparative novelty of the pipes and the excellence of playing (both of you!).

After watching these people play you feel you know them personally.
It makes up for the lack of narrative in song.

My €0.02's worth
Anahata