"First one that comes to mind is its length. Unlike the Gettysburg Address, this isn't some short speech that could be easily recited from memory."
You are probably not learning it for recital to prove how clever you are, you are presumably reading it for study and analysis.
"Secondly, there is a lot of religious imagery in the speech. I could certainly imagine people objecting to having the speech presented in a secular school."
On what grounds? On that basis you couldn't have US school textbooks with translations of the letters of Christopher Columbus in them -
"In conclusion, to speak only of what has been accomplished on this voyage, which was so hasty, their Highnesses can see that I will give them as much gold as they may need, if their Highnesses will render me very slight assistance; presently, I will give them spices and cotton, as much as their Highnesses shall command… and slaves, as many as they shall order, and who will be from the idolaters. I believe also that I have found rhubarb and cinnamon, and I shall find a thousand other things of value, which the people whom I have left there will have discovered…
This is enough. And thus the eternal God, Our Lord, gives to all those who walk in His way triumph over things which appear to be impossible, and this was notably one. For, although men have talked or have written of these lands, all was conjectural, without ocular evidence. ... So that, since Our Redeemer has given the victory to our most illustrious King and Queen, and to their renowned kingdoms, in so great a matter, for this all Christendom ought to feel delight and make great feasts and give solemn thanks to the Holy Trinity, with many solemn prayers for the great exaltation which they shall have in the turning of so many peoples to our holy faith, and afterwards for the temporal benefits, because not only Spain but all Christendom will have hence refreshment and gain."
- The conclusion to his letter to King Ferdinand of Spain.
"On that basis you couldn't have US school textbooks with translations of the letters of Christopher Columbus in them"
I don't know what your point is, most school textbooks don't include Colombus' letters either. And it's not that history textbooks can't include religious content but the more religious content there is the less likely you're going to see it in a secular textbook, grandparent is precisely right.
How on earth do you think you can you avoid including religious content in history textbooks, irrespective of the secularity of the school? You'd have to cut nearly all of it.
Besides I would have thought that a secular school would be one where you would see the widest range of religious content when studying history, as it would be less likely to be biased in favour of any religion in particular.
edit - the following is from a US lesson plan on Christopher Columbus for history teachers in public schools;
"In this unit, students will work with primary source documents written by Christopher Columbus around the time of his voyage to the New World, and with secondary source documents written at a later date. They also have a chance to write their own secondary source material in this unit. Using both primary and secondary source documents within a single unit gives students an opportunity to see the difference between these two types of documents. You may wish to have students work alone or in small groups to read and interpret these documents. Groups could be based on areas of interest (especially for analyzing the scholarly documents), or could allow students with stronger reading skills to help others work through the source documents."
and
"The desire to bring Christianity to native peoples was essential to European exploration. It is a topic that cannot be ignored in discussions of Columbus"
You are probably not learning it for recital to prove how clever you are, you are presumably reading it for study and analysis.
"Secondly, there is a lot of religious imagery in the speech. I could certainly imagine people objecting to having the speech presented in a secular school."
On what grounds? On that basis you couldn't have US school textbooks with translations of the letters of Christopher Columbus in them -
"In conclusion, to speak only of what has been accomplished on this voyage, which was so hasty, their Highnesses can see that I will give them as much gold as they may need, if their Highnesses will render me very slight assistance; presently, I will give them spices and cotton, as much as their Highnesses shall command… and slaves, as many as they shall order, and who will be from the idolaters. I believe also that I have found rhubarb and cinnamon, and I shall find a thousand other things of value, which the people whom I have left there will have discovered…
This is enough. And thus the eternal God, Our Lord, gives to all those who walk in His way triumph over things which appear to be impossible, and this was notably one. For, although men have talked or have written of these lands, all was conjectural, without ocular evidence. ... So that, since Our Redeemer has given the victory to our most illustrious King and Queen, and to their renowned kingdoms, in so great a matter, for this all Christendom ought to feel delight and make great feasts and give solemn thanks to the Holy Trinity, with many solemn prayers for the great exaltation which they shall have in the turning of so many peoples to our holy faith, and afterwards for the temporal benefits, because not only Spain but all Christendom will have hence refreshment and gain."
- The conclusion to his letter to King Ferdinand of Spain.
http://amhist.ist.unomaha.edu/module_files/Columbus%20to%20F...
"Finally and probably most importantly, what do speeches really teach us? They aren't particularly valuable outside of context."
What someone in the past said. Is part of learning about stuff, which is the context.