Re: ultimate reasoning behind prime directive ?
The PD is one of those things that could make practical sense if it weren't used chaotically and somewhat indiscriminately depending on the whims of the writers. The basic idea traced back to TOS "A Piece of the Action," where an Earth ship visited a planet before the creation of the PD and accidentally contaminated the culture by leaving a book on 20th century gangsters behind, and the culture copied the book. One can argue it's rather unrealistic to believe they'd have copied it as thoroughly as depicted and that it would displace whatever their original culture was so effectively, but it suits the plot.
"If you think you're brave enough to walk the path of honor, then follow me into the dragon's den."