A group of us proved a few years ago that Incredimail gathered information from emails and your pc without the users kowledge and they then sell this information to the marketing companies that sell on to those annoying spam email companies.
I believe that this is probably what was meant by being dangerous to you/your pc.
Even when uninstalled, Incredimail leaves spyware on your system and is difficult to get rid of without resorting to editing the registry.
Wow - Now that's what I call an Operating System!