That depends on a few factors. 1) The difference in the rate of your local bank vs Harley credit, 2) the amount you are financing, and 3) the length you keep the higher loan active.
Let's just use ballparks: If you can get 3% at a local credit union, and Harley is at 8%, then you are paying the difference of those 2 payments for 3 months.
Let's assume, just for example, that my payment was $200 more per month for those 3 months. So I'm "negative" $600. However, I was able to get an additional $3,400 for my trade, saved $1,600 in monthly payments on the bike I had for sale (the Mako I had on order wasn't due in until January at my local dealer), my trade would have been worth less, so I feel as if the $600 in payments gave me a benefit of over $4,400.
It all depends on your situation. For this particular deal, it worked very well in my favor, and the dealer moved a unit at MSRP, and will now be handling my referrals. Win Win.