First, I'd like to say that I'm loving the game so far. That said, I think that having a "stock market" in the game while it is so shallow is doing this game's brilliant concept and gameplay a disservice. Owning stocks should be worth something. In the real world the reason people buy stocks is (usually) not with a view to eventually owning the company, it's to create wealth from existing wealth. If buying a stock that then increases in value actually gave you some sort of reward other than simply inching you closer to victory it would allow another avenue of getting money. This would particularly aid players lagging behind. If you're in fourth place but you buy stock in a company that's doing really well, their success can help you compete. Likewise, short-selling (selling stock you don't have and buying it back later when it's cheaper) the stock of a company that you're about to underground nuke or that is about to go into debt at an auction, would allow you to profit off of others' misfortunes. 

The problem with this is that there would probably need to be other victory conditions, because as long as buying out another company is the sole winning condition there cannot really be meaningful stock-play. For example, the Railroad Tycoon series is one that, I always thought, gave an excellent stock market model that was easy to understand but also one that added an immense amount of strategic depth to an already deep game.