Just be careful that markets /= economy. The developing economies might still grow steadily, while their markets can fluctuate a lot. One of the most widely used Indexes for emerging markets is the Morgan Stanley BRIC. You can easily google and find some funds that invest in it and look at their performance to get an idea. The first one that I found (here) has a decent summary of its most important features. You can see that it is actually losing money . A very important number you should look at is the standard deviation, which is written to be 23% over three years. On the contrary, investing e.g. in the US health care sector has given much better results (see here) with less risk.

To summarize: what you say it's true in the long run, but equity investments have a significant short and medium-term evolution, which is generally independent of the long-term trends.