Many experienced traders will keep both long term investments and short term trades out at the same time. This allows them to play it safe with the longer term trades and buy strong long term companies while at the same time it allows a trader to attempt to make huge returns by trading the short term moves in the market.
Diversifying between long and short term trades can become what is the difference between even more important when you are just starting out. This way it makes it harder to make a few bad mistakes and lose all of your money.
The amount of money you put into long term vs. short term plays can vary widely. If you are very conservative you might want to have 80-90% of your account in the long term while having only 10-20% in short term plays.
What is important however is making sure that you keep these trades separate. Do not try to turn a long term trade into a short term trade or a short term trade into a long term trade. You may even want to open up separate accounts.
It is not a necessity to have both long term and short term trades but it can help you to have the best of both worlds. Almost any stock that is worth buying will be trading for more 10 years from now then it is trading now, and going after short term gains can often make higher returns then holding for the long term.