The idea behind stimulus is basically that the government will step in and take up the responsibility for the borrowing and spending that was being done by consumers, except instead of a Wii we'll get a high-speed rail line between LA and San Francisco, and hopefully the potholes filled in front of my house. At 0% interest rates, proponents argue, plausibly, that this borrowing is hardly going to crush the taxpayer under its onerous weight.
But that 0% is not on 30 year bonds; it's on shorter term debt that will eventually come due. What will our interest rate be when it's time to roll that debt over? It won't be pretty if the government is still having to fill in the output gap with heavy borrowing.
Fiscal stimulus has no great evidence of working (hell, we've only had so many recessions since Keynes even wrote the General Theory), and we're up a creek of massive debt (or more like a raging rapid) without so much as lifejacket. Adding on a bunch of debt that's sure to be mighty expensive to repay (and, keep in mind, interest is already one of the largest expenditures today) for something of questionable value seems like a surefire way to mismanage a trillion dollars.