Pleonasm (Not a Nasal Disorder)

Lately I have been noticing a lot of repetition in both written and spoken English. It strikes me as interesting that a language that favors conciseness has such a strong tendency to repeat itself. There is even a name for this useless wordy verbiage: pleonasm. As usually happens with things, the more I take note of pleonasm, the more it seems to spread. Well, actually, repetitiveness is not becoming more common, it's just that my awareness of it has expanded. For example, just a few days ago someone reminded me not to write "please RSVP" on some invitations, and that very day I received an invitation in the mail requesting that I "please RSVP" for a different event. Not long after that I saw some tuna fish in the store and I couldn't help but wonder: Is anyone not aware that tuna is fish? Then I heard someone preface a comment with: "for all intents and purposes." Therefore, with this barrage of pleonasms on my mind, I have started to wonder which ones are acceptable because of their idiomatic usage and which ones are errors? I mean, trial and tribulation, assault and battery, and cease and desist all seem like well established phrases in our language, and it is interesting to note that those I just thought of all have some sort of legal implication and/or usage. However, phrases like black darkness, repeat again, meet together, and reply back, just seem to me to be erroneous. And another thing: iterate already means to say something repeatedly, so why does everyone say reiterate? It drives me crazy, but reiterate is a valid word. 

So...I'd love to hear from all of you word junkies and language professionals out there regarding this post. When is pleonasm acceptable (in your opinion, or enlighten us with facts)? When do you frown upon repetition or actively seek to use it? Have you heard any other redundantly repetitive expressions lately?