I'm fine with the idea of being able to carry stuff over to a harder difficulty setting as long as said difficulty setting is balanced around that fact.
Impossible mode, however, just isn't challenging enough to still be difficult when you carry stuff over. In fact, the only difference between Impossible and Hard is that enemies do more damage on Impossible. It's obviously balanced with the assumption that you're starting fresh.
Personally, I thought it was a bad idea for them to allow you to both carry your stuff over to Zealot and get the achievement/trophy for it in DS2. At least they didn't make the same mistake for Hard Core.