"It's essential to have kept at least some of your powder dry."
I can't feeling this is strictly for mugs - I like to be pretty well fully invested.
I don't like my money to be hanging about and eroded by the moth and rust of inflation when it could be earning me juicy dividends.
Not many people - least of all me - actually know what the market's going to do.
If it goes down, I'm happy to get my dividends reinvest them at bargain-basement prices.
If it goes up, I'd be kicking myself for missing a buying opportunity.
If I'm fully invested, and some wonderful snip comes up, I can always sell something.
I may not like doing this, but, if it's not worth it, perhaps that snip wasn't so wonderful after all.