In their latest Kiplinger’s column, Whitney Tilson and John Heins examine a 10-year-old Fortune magazine article to stress some of their core beliefs about investing — including the idea that buy-and-hold investing (or, perhaps more accurately, “buy-and-forget” investing) can be a dangerous practice.

The Fortune piece, entitled “10 Stocks to Last the Decade”, highlighted 10 stocks that investors might add to their portfolios and be able to hold for the next decade, because these firms were poised to capitalize on overarching trends. Among them: Nokia, Morgan Stanley, Viacom, and … Enron.

All in all, Tilson and Heins say, an equally weighted portfolio of the ten picks would’ve lost 44%, almost twice what the S&P 500 lost during the “lost decade”. The duo says they point all of this out not as a critique of Fortune, but as a way to examine a number of important lessons. Among them:

Buy and hold isn’t what it used to be: Tilson and Heins by no means recommend fast-paced trading. But, they say, “buying and forgetting in a world roiled by rapid change is an increasingly risky proposition — especially when it comes to technology. … Now more than ever, you need to maintain constant vigilance in monitoring your portfolio.

Never underestimate the competition: “Although long-term demographic and market trends are powerful, a company still needs to have a strong competitive advantage to succeed,” they say.

Valuation matters: The average price/earnings ratio of the 10 stocks cited in Fortune’s piece was 100 — meaning that expectations were sky high for them. Even Oracle, which produced strong results earnings-wise over the past decade, stumbled stock-wise because it didn’t meet lofty expectations, they say.

Tilson and Heins also discuss the current market, saying that they think it has gotten too pricey, at 21 times trailing 10-year inflation-adjusted earnings. “Such a rich P/E is warranted only if the economy staqges a sharp recovery, which in our opinion is possible but not likely,” they write. They say they are shorting more stocks, and focusing their long portfolio on blue chips.