Personally, I think that bards should have the ability to create artworks. I haven't had much luck in creating rules for this in the past - but with Daniel's help, I have an new angle on the process.
It basically works on the principle that while art can have a strong effect on a person's consciousness, this effect is often temporary. For example, I remember many times when a particular song came on the radio (one I didn't own) and my world stopped. I would stop whatever I was doing and just listen. Music can be all possessing, particularly when it speaks in a particular way about what's going on around us. It can seize hold of some people - including me - and make us dance in public, not caring what anyone else sees or thinks.
But it also passes. That image on our puter, the art we bought at the convention six years ago, those things are still precious; but they don't have the same impact they had once. In a way, art is like a magic spell in slow motion, with a very long casting time and an equally long duration . . . but like any spell, the effect eventually dissipates.
So what is needed is a set of rules that compares the level of the bard with the amount of time taken to produce the artwork, which is then used as a guideline to determine the specific effect of the art, how much and how long.
Take one of Daniel's suggestions, that the art shortens construction time. We can imagine a set of workers building a temple who have already seen the statue that will feature as the temple's central focus. Enamored with the statue, they are encouraged as a group to work doubly hard and more congenially to complete the structure, to protect the statue, giving it a home and pleasing themselves.
If the building is itself an artwork, then this can have a similar effect on the town: increasing the honesty of the residents, increasing their productivity, decreasing crime, encouraging martial spirit, expanding donations to the poor and the church, as well as thinking of the authorities with inheritances, causing a whole region to become more powerful economically and militarily. All from one statue promoting one building. This would help explain the whole period of cathedral building in Europe in D&D Game terms.
There's also the actual development of the work to be considered. The cathedral is impressive each time an element is added - it doesn't depend on the whole work being finished (so long as the work is evident and ongoing). The same is true of the statue; visitors could go and look at the progress of the statue each day, seeing elements that improved their lives, similar to the way we see serial artworks like television or movies with sequels. Therefore, unlike spells, the casting itself breeds a sense of positive expectation, like players anxiously expecting the moment the mage will get into position and readiness to launch a given spell (an tension-building experience that was completely destroyed by the fast-food style of spell implementation that came with later game editions). With really great artworks, this is something to consider.
I should also think there need to be rules about experience with art when long periods of time have passed. Used to be, before the internet, when I would only hear a favorite song or see a favorite movie with months, even years, of denial. The longer the denial, the more powerful the re-acquaintance. We need rules for that - and those rules still have to take into account that even if we haven't been to Mecca for ten years, we have been there before . . . so no matter how good the second experience is, it won't be the first experience (unless something has changed about Mecca).
Those are a lot of details to be worked out, without a lot of random tries being attempted and a lot of game-play mistakes made. But this is what I love about game design. We could do this! How? No idea.