Is one level of Bard generally worth it on a single character in your party, just so that character can use the Bard-only items in the game, or are you better off without that level?

Well, with one level you'll get the dialogue... As for the instruments, let me just say that, as with the armor and weapons, some depend on alignment. If you consider dipping into bards I suggest that you take enough levels to cast second level spells (mirror image). Doing this will also assure that you have plenty of points in those oh so lovely speech related skills.

About those "speech related skills," do Bluff and Intimidate mean anything? I just have a feeling Diplomacy is the most important one of them all.

I know of places where each skill has been used. Ok they are few and far in between but none of those 3 feats is more important than the other. Taking 1 level of Bard just for the items is perhaps not worth it. Also some of the dialogue that Cr mentioned require a minimum Bard level.
If you have nothing better to do than take a Bard level, go ahead. Otherwise I don't think its of much use.

what bard only dialogue is there?

I would say that if you have 6 party members then it is absolutely no loss to take one measly Bard level. You wouldn't even notice the loss of another class level and there are enough Bard only benefits to make it worthwhile, esp in Heart of Fury mode with some of the cool items.