You are right, the downside is very small (depending on the strike you buy), but the upside is limited as well - you need the stock to increase in value by the price of the put just to break even. So the stock can go up 20-30% and you are still up only 5-10%. As usual, there is a tradeoff to any strategy.
It is true that synthetic stock requires less margin, but I would be very careful to base the position sizing on margin and not the stock value.
For example, to buy AAPL stock, you will need $13,300. So lets say you have 13,300 portfolio and buy 100 shares, no leverage.
Now lets say you buy a synthetic position that requires only $3,300 margin. If the stock declines to $100 (25%), your loss is $3,300 or 100% on margin. If you purchased 4 contracts in your $13,300 portfolio (which you could do based on margin), your account would be toast.