I know that writing out one's sermon is a cherished idea. I suspect it starts with homiletics class, where we wrote the sermon out in advance, so the professor could critique the sermon. The problem is that guys keep doing writing them out. And woe to those who deviate from the written sermon. Supposedly, this is a path to rank heresy.
The problem is not with the written sermon itself. It's what it does to the preacher and the preaching style. We all know those guys who talk in a perfectly normal voice over coffee but develop a stained-glass voice for sermons. Not an interesting, beautiful voice, but what they hope to be A Holy Voice. And what they do is end up sounding pretentious.
I honestly think that pulpits are also part of the problem. It's not uncommon in Missouri Synod circles to mock those who preach outside the pulpit as "chancel prancers" or the like, but let's objectively look at what preaching from the pulpit suggests. It looks like a lectern, and is likely to make the preacher think he's lecturing. And the congregation becomes -- rather than the sheep being fed by the shepherd -- a group of students, dutifully hearing their weekly dose of religious instruction.
And a church is not a classroom. A sermon is not a lecture. Celebration of the Eucharist shows that we are giving forth life, and faithfully -- per the Augsburg Confession -- giving and receiving Christ's Body and Blood is one of the best ways to reinforce that church is not -- per Melanchthon -- a "heavenly academy."