Timing is a zero-sum game before transaction costs and taxes are factored in. After taxes and costs, the vast majority lose over long periods. I don't like those odds.
If it were possible to rely upon macroeconomic indicators to predict market returns, everyone would act upon them simultaneously and they would no longer be reliable.
What Bob says here is true for all investors as a group. They can't beat the market because, by definition, they are the market. But, somewhat paradoxically, the thing that makes the market as efficient as it is, is that a significant number of participants are always trying to beat it. I'm one of those myself, to a degree, in that I keep the bulk of my assets in low cost stock and fixed income investments, but do some "active" or "tactical" shifting of my asset allocation in response to macroeconomic conditions. Specifically (but greatly simplified) my long term target allocation is around 60% equities and 40% fixed income. But at times like the present, when I think that within the next year or two the prospects for equities are better than for fixed income, I shift that to perhaps 65%/35%.
Without going into a detailed explanation, I think that the benefit that I get from this is a reduction in portfolio volatility for a given long-term asset mix. I can therefore invest a somewhat higher long term average proportion in stocks than I otherwise would. (Actually, a good part of this is in "equity equivalents" such as high yield bonds and REITs, which I believe reduces volatility even more.)
In several posts I have recommended the weekly column on personal finance by Jonathan Clements in the Wall Street Journal. One of the rare cases in which I slightly disagreed with him was over the benefits of asset reallocation. He says that it reduces portfolio volatility and "may" increase return. I agree that it is beneficial (especially for retirees) in that it reduces portfolio volatility, but for most people will only increase their return if they are lucky enough to do their reallocation at precisely the right time -- that is, when the returns on one asset category relative to another are at a peak. For many people, asset reallocation actually reduces long-term returns by preventing their asset mix from shifting towards the higher returning assets (especially, small cap stocks).
For example, when stocks were booming in 1999 and 2000, I was cutting back on them. In retrospect, that may look brilliant, but at the time it meant that I wasn't getting the full "benefit" of the run-up in stocks. When stocks eventually dropped, the "paper value" of my assets lost less, but there was less to lose. What I did accomplish, however, was less volatility, and thus less disappointment from the market drop.
So even if you do the "right" thing and lighten up now on long-term bonds, you won't know precisely the right time to shift money back into them. And people who "park money in cash" waiting to see what will happen will experience less volatility, but will also experience lower long-term returns.
