The current setup gives game developers a somewhat predictable revenue model and on the strength of that model they can encourage investors to put up money to make the games. Changing the revenue model may change how investors look at the market. Maybe for better maybe for worse. Who knows, but it would be a disruption if Steam has to adopt this model worldwide.
The thing that I don't like is if you allow reselling then developers get less long-term revenue but (hopefully) more short-term revenue. This will cause them to focus more on getting a big opening weekend than making a solid game that might have years of appeal.
Portal is still $10 on Steam. Maybe they only sell 100 copies a month at this point but that's still money coming in rewarding them for investing in interesting core game mechanics. I'd rather see them get rewarded for that than for dishonest advertising campaigns because 50% of their money will come from the first few days of sales.