You need to balance your investments. Putting in too much money into a stock might cause you to lose more money than what you can afford. There is always a chance that an investment will fail. The last thing you would want is your portfolio shrinking dramatically to where you can no longer make a sensible investment.
However, you cannot afford to just make cheap trades all the time. You have to spend a decent amount of money on your trades if you want to make something sizable from your trades. Anyone who spends little is surely not going to reap the benefits of being a successful day-trader.