What exactly is the down side to this?

You should re-think your retorts a little better next time.
Let me show you why......
This sentence shows two things about you: 1. You're a tree hugging liberal, and 2. If you want to be taken seriously in any debate, you should use that tool called "spell check".
First of all, just WHERE do you suppose those good ole' "once-a-year" checks come from????? Let me give you a hint.......it's called a REFUND for a reason. Dur, durrrrr......

It's money that you've OVER paid. It's NOT "free" money. It's giving you back a PORTION of what YOU have paid AHEAD OF TIME, ABOVE AND BEYOND what you were "supposed" to. Follow that? Probably not. Well then keep thinking that the IRS just "gives out" all those checks once-a-year.
Second, how many checks do you think that "Americans below poverty levels" get NOW????? You sir have no clue how things actually work. It sounds to me like you've been indoctrinated rather well into the Democratic way of life.
Again, you have no clue what you're talking about. Those "Conservatives" you incorrectly quote probably want people to stand on their own two feet rather than ask for hand-outs from good ole' Uncle Sam. Yet again, I fail to see the "down" side to this thought process.
Why do you keep repeating the same old tired war cry of "poor people are going to suffer"? I'll tell you what......explain exactly HOW those "poor people" are going to suffer? I challenge YOU to come up with YOUR own thoughts on just HOW the Fair Tax is going to make people who PAY LITTLE TO NO TAXES NOW pay any more than they are already paying......which again is LITTLE TO NOTHING. Let's see how YOU answer that.
Before you spew out incorrect information again, educate yourself better about what you're trying to put down. You sound exactly like that Texas Senator that looked like a deer caught in headlights when he was asked to name ONE of Obama's accomplishments on National TV. I bet you've not even read the Fair Tax book, have you?