[NathanKP]

The Prime Directive only works if you assume that these hypothetical species are much more altruistic than we ourselves are. I find it highly unrealistic that any species, however highly evolved, would be 100% altruistic, with no rebels/thieves/slavers/prospectors that would desire to interfere with or destroy us humans.

And that's not to mention that realistically it would be almost impossible for a civilization to completely hide its presence. For example as humans our radio footprint from TV and radio and satellite broadcasts is spreading out of our solar system. That's going to keep on traveling and I don't see any way that we'd be able to just "erase" that so that no one else would notice it.