I did not grow up in a tradition that used or truly valued the use of prayers written by others. The only exception would be the Lord's Prayer which was used because, of course, it was written by Jesus. The spontaneous or "unique" prayer was what I was taught, learning to pray in public, putting my thoughts to God into words to be spoken, on-the-fly aloud in a group. Although I don't think anyone actually ever said this to me, I grew up thinking that this type of praying must be superior to reciting prayers or even pre-writing a prayer to pray outloud at a later time. And this type of thinking has engendered a type of pride within me because I know how to pray aloud, on the spot.
Today as I was listening to an interview with N.T. Wright, something he said nailed me between the eyes regarding this pride that's been growing in my heart over the years. He says that to use the prayers of others is actually a sign of humility, allowing someone else's words to express some longing within my own heart and learning from them. He further encourages that if the Holy Spirit helped them to pray in that fashion, why not "ride in on their coat tails." 
These "old" prayers can serve as a structure, not that restricts or boxes in my prayers. Rather, they give me a framework from which to start, a springboard into areas of prayer and meditation that might not have occured to me before, offering me a fresh perspective of the Lord and His work in my life. The very speaking of them become a form of praise as I join in with another's expression of worship.
In these last couple of years, I have been discovering these ancient prayers and their writers and have been so blessed by them. But, to be honest, a part of me has struggled with using their writings and prayers in my "quiet time" because somehow I felt like it was less authentic since it wasn't spontaneous and uniquely from me.
Like a good friend of mine says: "We sing other people's songs. Why not pray other people's prayers?" I am looking forward to this next year, to discovering the prayers of the ancients (anyone older than I am) and being blessed by the move of the Holy Spirit in their lives.
1 "Reclaiming Worship: a training interview with N.T. Wright"