When you open a bank deposit, banks would like you to keep your money there for very long-term. They will give more interest for your money. Often, your bank account will only shrink with time. The rate of inflation often greater than the interest you get.
When you turn to "professional brokers" (read "sales people"), they commonly would like to keep your money for at least a year.
When you turn to funds, or invest in your pension. They will explain you theories why long-term investments are in your interest.
Even better, they will try to get a cut of your salary or pension each and every month. They tell you about theories like "money averaging".
Money averaging is actually workable theory. Let's say you buy the shares of Nokia each month. You invest each month 100 dollars. When the price of Nokia is up, you get less Nokia shares, when the share price moves down, you get more shares. An average, it works out quite well for you.
Another theory supporting long term investment is the theory that if you have only missed 20 days of the last 10 years of the stock, you would miss the majority of the profit. Scary, isn't it? Loosing most of the money just because you missed 20 days in the stock! Better to keep your money in the shares, isn't it?
Being scared of the loss is exactly the purpose of the "Wall-Street", banks and institutes. They would like to keep your money. And they have millions, billions for marketing, persuading, selling you to keep your money with them.
They hire writers to write books. They are the sole financing behind financial newspapers. They are on TV commercials, news. They have politicians in their hands. Arthur Levitt, former chairman of the Securities and Exchange Commission, wrote an excellent book on the subject "Take on the Street". Even groups whose purpose is to protect the individual investors are organized, financed by these financial institutions.
I don't think asking the question: is long term good or bad relevant. Long term does have its place.
The question really is: whose interest it is to advertise long-term investment so much for us?
Did you know that banks, the wall-street, your brokers are making tons of money by short term investments?
Many believe that long-term is safe, while short-term is dangerous. So the question is: who is advertising this idea?
Did you know that brokers are allowed to use the shares and money that you keep on their account? If you don't believe this, just check the contract you need to sign when opening an account with them.
Did you know that many bank traders only hold short-term positions?