Stocks are only useful for two things:

1. You are relatively new to the game, and are having hard time reaching the number of levels required for the next company reset. Essentially, you need at least another upgrade for your 11th girl, and it will take hours. While you are waiting, you might as well play on stock market to multiply your money and speed things up a little.

2. You played for a while, farmed a lot of company levels/shares, and now the cheapest decoration upgrade costs crazy amount of shares (100 million+). The only way forward is to go for an uber-long run using the stock market. This grows the shares exponentially. The exponent is pretty slow (means it takes several days to double the shares). But if you are patient and play for several months without resetting, you can get an insane amount of shares.

Coffee is pretty good - it is actually more efficient than Gold and Diamonds. The disadvantage is that you have to constantly babysit it, buying/selling every five minutes.