I may just end up buying Heart Gold, but there's a chance I'll buy Soul Silver as well. The main reason would be the same as several people stated here – one for my "main" game, and one for replaying. Also, I'd like the chance to play as both the male and female main characters – I would use the female character in HeartGold, and the male character in SoulSilver.
Another reason is because I'd like the chance to choose two different starters. I'd pick Cyndaquil in HeartGold and Chikorita in SoulSilver.
Lastly, the games would be useful for obtaining version exclusives and for evolving Pokemon that can only evolve through a trade. I don't have Wi-Fi and probably won't be getting it any time soon, but I can borrow a DS from one of my sisters so that I can conduct the trades myself using the two games.