Let's take a look at the 2000 election results for Santorum. He ran against an anti-choice Democrat, Ron Klink, who despite being a fairly bad candidate and having no money only lost 53-46. But, where did he lose?
The 4 suburban philadelphia counties are critical in PA politics - Bucks, Chester, Montgomery, Delaware. In these counties, Gore won 51-46.
Santorum got 565083 votes in these counties. Klink got 419439. Leaving off the small party votes because I'm too lazy, that makes it about a 57-43 race for Santorum in these counties.
Basically these counties have a lot of middle of the road pro-choice voters. And, in an midterm election you're going to be asking them to hold their noses and actually go vote for an anti-choice candidate.
If Casey can't win and inspire large turnout in these counties he really can't win the election. A contested primary would give him the opportunity to figure out how to appeal to these voters.
Every true believing conservative resident of Pennsyltucky will get out and vote for Santorum. Can Casey inspire voters?
Generally, I think the uncontested primary idea is a really bad one for Democrats, though they still cling to it. So, I'm glad that Pennacchio at least is still in the race, even though the press is trying to pretend he doesn't exist.