Probably not, but all of the other small income-based social programs add up in a hurry, especially for people who are on the bottom end. If you know that working more will cause your earnings to rise just enough to knock you over the line and end the free healthcare, fuel assistance, food stamps and all the other programs, its a huge disincentive. What's the point of working harder, if it means you're going to have even less money than if you didn't work at all?
Sure, at this point it's pretty well understood that it is bad to build programs with regressive structures. I think the phrasing of the OP threw me off, I had thought they were saying that payments in raised the effective marginal rates, not the loss of the benefits.
