For what it's worth, I'll tell you that my 2001 Sienna (bought new) also came with a towing package. Indeed, every Sienna on the lot when I bought mine had the towing package. After learning about the infamous Sienna oil sludge problem, I did a little investigating and came away with the distinct impression that at the time I bought my van and, apparently, still in 2003, Toyota (who know about the sludge iussue but had not yet owned up to it) had begun shipping all Siennas with the towing package, believing that the larger radiator, etc., would help the engines run cooler and thus ameliorate the sludge issue. Someone more knowledgeable may want to comment (perhaps I'm way off base).
I will tell you that I am still driving my Sienna nearly six years later (approx. 90,000 miles) and I've had no problem with sludge and I've experienced no negative effects from the towing package (even though I never have pulled a trailer with the van). Again, take it for what it's worth...
Kevin