It gives you a cheap, small damage sweeper in the early game, or you can foretell it to be a more impactful sweeper later on
Since all Modern cards should pass through Standard at a given point, WotC was having trouble feeding the format with new cards or reprints, since cards like Tarmogoyf and Cryptic Command would be too powerful for Standard (were talking about the 2013 MTG era, keep in mind)
So, a Ruby/Sapphire card that provides an ink ramp (a Sapphire hallmark) can be placed in a Ruby deck, providing more options and versatility
If the creature Audacity would enchant is an illegal target by the time the Aura spell resolves, the entire spell doesn't resolve
It won't copy any counters or stickers on that creature or any effects that have changed its power, toughness, types, color, and so on, and a creature becoming a copy doesn't remove any such counters or effects that are already applying to it