Take this year, for example. I happen to have a mortgage that I could pay off completely later this year without penalties. I've been saving to do just that, and parking the balance in the market until the day comes to pull the trigger. But what's this? I'm up 11.6% on the year thus far. It's hard to come up with an argument for not letting it ride a few more months and watching to see what happens.
Granted, there's always risk in the market, and using the fund today would net me a cool 4% risk free. But still...