We used merchant defined form fields to carry information back to our site after AN processing, rather than using session variables to maintain state. We have just switched from using AIM and notice that with SIM or DPM all those fields come back at the bottom of the merchant receipt just as you would expect. However, some of those fields are related to the application and its needs post-transaction and are not needed on the receipt.
If would be a nice additional feature in the API to have options to not show those fields in the merchant receipt. For example, all those form fields with a name beginning with "Y_ "could be omitted from the receipt while all others remain in the receipt.