I see your point, but I still don't quite see how you justify the repetition when Jesus specifically said not to. I will agree that many people aren't verbally inventive, but my belief and understanding is that prayer is conversation with God, and being that, should be of a more conversational nature, similar to how we speak with each other (with the proper respect shown to God, of course).
If I need to talk with God about something specific (or even not so specific, as in I don't know quite what to say), I can't see how repeating over and over again the Hail Mary or the Our Father is going to really accomplish anything. It would be more likely to put me in a trance, and that I have a real problem with, as I don't believe Christians should put themselves in a trance. It's quite another thing if God does so, because His protection would be a part of it. Putting oneself in a trance would seem to me to be dangerous, because of the potential for opening oneself to demonic attack, because it is a voluntary lowering of ones mental defenses.