The tenuto sign has become almost the equivalent of the semicolon in the world of written music, meaning we all feel it has a function, yet precicely what that function is, tends to be a matter of personal preference. To me, it has to do with weight; I employ the sign when I wish to add prominence and importance to a note, yet not blatantly or loudly, the way a marcato sign can imply. If I wish for a tenuto-marked note to be held longer (though still not quite the "length" of a fermata), I will add "ten." above the sign. Slurred tenuto has to do with a slight separation between given notes, yet not quite so detached as a slurred staccato would perhaps indicate.