That's exactly right, tunalover. We looked at IcePak and Flotherm - great tools but overkill for what we need, and a major investment in $ and time.
An engineering tool is just that - a tool for engineers. It will be used by EEs with some experience in heat sink selection.
You are also correct about the limitations, though I never met a tool that was "too easy". HeatSinkDesigner is limited to natural convection and "parallel flow through plate fins" which covers most of our applications, but not all. They offer a free Flotherm model of any sink their tool can't handle. I intend to test this claim soon.