Story mode is not dampened if you don't have R/S. You just can't play the Colosseum part of the game until you purify 4 pokemon in Story Mode. After you get a good team in story mode, you can register it in Colosseum mode and play that to your heart's content. If some of your friends have R/S and a GBA to GCN cable, you can play multiplayer, you with the story mode team and your friend(s) with their advances. Note, you cannot use the GBPlayer for this purpose unless you have 2 GCNs. Reason being that the GBP turns the GCN into a GBA, not allowing for GCN Games to be used at the same time. Nintendo could have included the drivers on the Colosseum disk to make it work, but didn't. Oh well...
After you beat Story mode you can trade with R/S, you may want a GBA for this, but it's not needed to enjoy story mode.