Because someone has to do it! A couple of people, including my beloved colleague Chris, have ridiculed Penn for pointing out that Obama has "never faced a credible Republican opponent," when the worst direct challenger Hillary ever faced was the not-so-fearsome Rick Lazio.
That's true as far as it goes, but Penn was making a broader point. Here's the line in context:
Sen. Obama has never faced a credible Republican opponent or the Republican attack machine, so voters are taking a chance that his current poll numbers will hold up after the Republicans get going. With Hillary, the GOP has already tried just about every attack and has failed. Those attacks are already factored in her ratings, where she remains competitive against Sen. McCain. But when it comes to Sen. Obama this is a big unknown, and the likelihood is that his negatives will rise.
Penn's making a point about, basically, the "Vast Right Wing Conspiracy," and its years of attacks on Hillary--not electoral challengers like Lazio. I don't find that crazy or misleading.
Obama can certainly answer that his strong performance against the (once) mighty Clinton machine argues well for his ability to weather the GOP storm. But the dynamic in a Democratic primary offers limited evidence about how a general might play out.
Personally, I always thought it possible that Hillary's negatives would drop in a general election: She's become far smoother in the spotlight since the mid-1990s, when many folks learned to hate her. (Although I can't say she's done a stellar job on that front in this primary race.)
I do think Penn's right that we have little idea of how effective the GOP attack machine would be against Obama. I'm not sure how anyone can predict that. But it doesn't strike me as an absurd question and, now that Obama's the clear frontrunner, it would be interesting and worthwhile to examine it.