Most often one of the biggest failures of a manuscript speech or scripted speech is the lack of intonation that comes from reading. The very nature of it tends to make people seem robotic. The other is that it may not sound natural as it may not reflect the way in which someone would normally communicate. Consider someone reading a script written by a physicist to a group of children.... Not very effective.
Unless the speaker has a very dynamic personality capable of infusing appropriate emotion into his/her delivery while, at the same time, holding the attention of the audience, a manuscript speech can very quickly become a disaster. The manuscript, by it's very nature, is meant to be read. As such, it is not only possible, but likely, that the reading will be boring and the audience will lose interest quickly. The average reader of such a script tends to follow it word for word without getting a feel for how the audience is receiving the words. Good communication requires a sort of back and forth relationship between the speaker and his/her audience. A good speaker will make adjustments throughout the presentation--a tactic that is all but negated by a strict manuscript. One exception to this is if the speaker has memorized the script. This allows the speaker the option to deviate from the script in his/her delivery, yet retain the message throughout.
There are way too many factors to consider regarding this issue to cover in one short answer. There is a mountain of information out there covering public speaking and communication. But, one doesn't need a book to recognize who the really good speakers are around the world.
My personal historical favorite: Winston Churchill. He toiled over his impending speeches. Wrote them down, word for word, then memorized them. He spoke to many a miserable reception throughout his life, but when the time came for him, and only him, to deliver with the immense weight of his country and the world upon him, his words helped change history and are still powerful today.
If one must prepare a manuscript for a speech, one may as well make copies of it to hand out to the audience so they can read it on their own time.
A Manuscript tends to keep the speaker's attention on the words on the page, rather than on the actual concepts he/she is talking about. The audience tends to capture the speakers thoughts and imagination. If the speaker's thoughts and imagination is on the manuscript, and not on the issues and concepts he/she is addressing, the connection with the audience is lost. I hope this helps.
