There is nothing inheritly wrong with spyware doing market research (such as monitoring your activities for the sake of improving the targeted advertisement), but its the uncertainty of what exactly they are doing. The over all flaw of closed source is that the user has no idea or an easy way to find out, what is being place in their computer. Like for example no one can say for sure if Bearshare or Limewire themselves are free of back doors, the only ones who would know are of course the developers for Freepeers (Bearshare) and Limewire. From that point on its a matter of do you trust these companies and more specificly their programmers, do they seem like responsible people, or do they seem like the kind of people who will put back doors in their software (for what ever reason). It may help others to know, but there has not been a known gnutella client to have a back door in it (that does not take into account the unknown), so its less likely that there is one. Bearshare is the closest one to come to that point, when they forced people to install spy/ad-ware, but they changed that policy.

Just remember this is about trust and responsibility.