Many studies, for example in TASC, Technical Analysis of Stocks & Commodities (December 1999), showes that buy and hold has high advantages compared to trading only an index or a few stocks. The reason for this is that the profitable moves occurs in bursts and is easily missed if you are not in the market when they occur.
That is, on average we have 250 trading days a year on the market (a fairly stable number). On average, if you take the best 32 days of such a year, you have achieved the vertical price gain that the market has experienced during the whole years 250 days.
There will be price movements the other days as well, but if you confine yourself to an index or a single stock, on average the days will not achieve any net change. The shorter term you use for your trading, the bigger is your chance for finding profits also during these 220 days. For example a day-trader can make many deals during one single day. But for the more intermediate term trader, these days will not be beneficial unless you look at many different stocks, utilizing the cycles in the market, finding the merging possibilities in different stocks.
If we utilize all stocks on the market, the statistics above can be turned into totally different figures. For example, looking back to 1989, there has not been a single week that has not provided a lot of possibilities to make money on stocks. There has only been three 14-day periods during the last 10 years where there have been as few as 15 possibilities within the period to buy stocks and win at least 5% in 10 days. All other weeks has provided a lot more chances. The diagram below shows this.