I don't see how really ? cuz also everytime you close/amend my long term position for the sake of daytrading ( 20 times a month ) you are committing yourself to reopen/reinstate my original long term trade , remember i want to stay long the SP or the USD/Yen for example as a position trade or as an investment ....
Consider this scenario: You are long 50 ES futures as Shakone suggested, and you get a short term bearish trade which puts you 5 lots short. Then, while you have these positions on, for whatever reason you decide you want to close both trades (i.e. be totally flat - the reason is not important).
By adjusting your long 50 futures position to only being long 45 contracts, you have to sell 45 contracts to go flat.
Or, by holding 50 lots long and 5 lots short, you have to buy 5 lots back and sell the other 50 - thereby trading a total of 55 lots.
So to get the same result (being flat), you can either trade 45 times or 55 times. Doing 55 trades will always cost you more than doing 45 trades, be it in the form of the spread or commissions.
Whether you are long 45, or long 50 + short 5, the P&L of the trades are identical. The difference - and why one approach is better, assuming you want to make money - is that circumstances can arise (as I've explained here) where you have to do more trades than are necessary.
It doesn't matter how liklely or unlikely you think this situation is, the economics behind the transactions make one more desirable than the other. Nobody rational would expose themselves to more costs - or even the possibility of more costs - than absolutely necessary. By having the two positions on rather than adjusting your existing one, you are exposing yourself to the possibility of:
1) Additional trade costs
2) Additional financing costs
3) Opportunity costs
This is why, if your aim is to make the most (or lose the least), you should amend your existing position rather than open a new one. It is
de facto better for your account balance.