Unlike other mobile phone manufacturers, Apple has always stuck to a single new model every year since the iPhone was originally released in 2007. Other companies may release five or fifteen models a year, Apple sticks to just one. However, it has elected to maintain older models up to two generations behind every year and sell them at reduced prices. As a result, it can hit lower price points that it could not in previous years and provide competition to newer and cheaper phones. After all, many new phones being sold at lower price points essentially use older technologies anyway.
The 3GS currently is sold in the US for either free or $1 up front with a two year subsidized contract, but in developing markets, the phone sells for roughly $375-$430 without subsidies. Consumers in developing markets also tend to purchase phones outright in the first place either by choice or by design. Mobile telcos in these markets don’t necessarily offer subsidized contracts.
By offering the 3GS at a lower outright price even as Apple releases a new iPhone later this year, it could significantly increase the adoption of iOS and provide Apple with an even more expanded lineup without compromising on size and quality. Additionally, if Apple does offer the 3GS alongside the iPhone 4, iPhone 4S and the upcoming new iPhone, there might be a chance that Apple maintains compatibility with the upcoming iOS 6 even if it may be in a limited fashion.