Is there a logical reason the income limits are set so low ($0-$10,000) for phaseout of the ability to make a contribution to a Roth IRA for married taxpayers filing separately? Where is the potential for abuse?

I certainly have no answer. Without reading the JCT "blue book" on the legislative history it's hard to determine. And I'm only guessing here, but the "Married filing singly" treatment is similar (the zero to 10,000 restriction) on regular IRA's, so that may simply have been carried over to Roths without a lot of thought. Possible it isn't even addressed in the blue book dealing with Roths, so you'd have to delve back into ancient history of 219(g) or whatever.

Probably just enough to know that the restriction exists. The "why" might just raise your blood pressure!

A useless answer, I know, but it's all I've got.