Why compromise with Republicans if they won't vote for the final bill anyway?

One of the great mysteries of Washington DC these days is why Democrats spend so much time negotiating with Republicans. I can understand negotiating with them if it gets some of them to vote for a bill, but that hasn't been happening. So why bother? Or why keep the Republican compromises in the bill once it's clear Republicans won't vote for it? I understand the notion of wanting bi-partisan cover for a bill (though I think that notion is highly over-rated), but when it's clear that's not coming, why let them contaminate the bill? Do they actually think Republican input makes the legislation better? Ezra Klein wants to know the same thing, and speculates as to an answer:

Republicans aren't just getting some technical amendments passed into these proposals. They're helping to design the entire architecture. They're securing long negotiation processes that give them much more input than your median committee Democrat. And then they're abandoning the final draft. The underlying dynamic here is pretty simple: Because so few Republicans are willing to be bipartisan, there's no competition from other Republicans to be the guy with the commemorative pen when the bill is signed. That means that it's harder for the Democrat to walk from one Republican and find someone else, which in turn means the Republican they are negotiating with has a lot more leverage.

The result has been final, partisan products that are much more compromised then their bipartisan predecessors. Medicare got a dozen or so Republican votes and was a single-payer health-care system. The post-New Deal financial regulations were also more bipartisan, and were also much more aggressive than what Dodd is proposing. But the Democrats behind these compromised pieces of legislation don't get any credit for compromising. Instead, they just get hammered for all the weaknesses in the bill -- weaknesses that were added to secure Republican support that never materialized.

The interesting question is why Democrats don't toss out their compromises when they lose their potential co-sponsors. The answer, I think, is a mixture of conservative Democrats and industry, but it's hard for them to explain that.