A couple of thoughts on this issue... (And I am not "for" canned sermons.)
First, who is preaching to the preacher? It is important that the preacher also be challeneged by someone else who preaches the word to him.
Second, which preacher in today's church has the time (as Jim has so elequently written) to do all the messages and lessons that any given church demands of him? Not that they do not try.
Third, some sermons are worth re-preaching because they are so well done that God uses them over and again. In this sense, EVERY preacher who follows the Word carefully will preach at least a few "canned" sermons in his career, for he will ultimately preach a sermon recorded in the Word, one of those "so well done that they bear repeating" sort of texts.
And, fourth, it is good to introduce new styles and material into the pulpit ministry to keep the preacher fresh. As he reads or hears others preach he gains insight that helps his own preaching ministry.
To wrap up my thoughts, one thing that I've seen over my years, both as a preacher and as one listening to preaching is that most preachers don't REALLY write a new sermon for every preaching event. Hardly... What they do is "fill in the blank" of an existing sermon format that they repeat over and over again until the church finally sickens of hearing the same expressions, the same illustrations (which are also canned, and about which a lot of preachers become disingenuous -- who has had THAT many evangelistic incidents while flying on an airplane?).
I've been able to set my watch by what some preachers say at this time or that in any given sermon. They are predictable, and just preach the same thing with different names or context. That is all they seem to know. And, most don't really have the luxury of gaining new insights, for either time sake (golf is important, you know) or because they don't really care to stretch theologically.
It is not that difficult to set apart the ones that do the repeat thing... Just ask them what they've been reading lately and look for the blank stare before they spit out the approved Sunday school answer, "Well, a, duh, the Bible. It is ME AND JESUS all the way!" Except that they haven't "grown" in their exegesis or understanding in decades.
Now, on to me... Wish I could set myself apart from the crowd, but I am under the same time constraints as everyone else. I get 24/7 and no more, and I am a bi-vocational pastor who has to balance 60 hours a week on a secular job with church life and preaching.
I do not use "canned" sermons. I write each one, and lessons for the church as well, but I certainly do not either take 20+ hours a week to do so. Is my work "shallow" or non-biblical? Nope. I study while I work. I study while I'm here answering posts and challenging my theological thinking. I study every time I crack open the Word to deal with a good defense, or just to read my daily reading. I look while I work to find the good examples I need to illustrate my sermons (never use those books of illustrations!). I change up the sermons, styles, series, etc. And, I do that all in under 10 hours a week with modern computer study aids that allow me to do a background investigation of my preaching text that most would never begin to dig out if they had to consult book upon book to do so.
I generally read at least 3 major commentaries on the passage, I read it in the original language, diagram it in the original languge, then parse it, looking for the keys that make this exegetical effort fresh and applicable to our culture and needs (which are mostly similar to ALL cultures and needs, as we all struggle with the same sins, etc.). I begin fleshing out the text by mid-week, and manuscript it on Friday or Saturday, then read through it making subtle changes and notes, inspirations, etc., before final reading Sunday morning before church. Then, I stand up and preach -- I reference my notes for quotes only -- and what I say may or may not be precisely what I wrote earlier, but it edifies the body, it is fresh and new, and it challenges as I deal with "so what" questions.
I am preaching 1 Pet 2:1-8 this Sunday. Fresh and newly written sermon. I'm also looking forward to Black Heritage service that we have in mid-February (can find it in the text that comes next!) and a sermon that ties in Valenties Day (can also find that in the text!) after that.
I will teach a series based on membership lessons from Mark Dever's church (Capital Hill Baptist) on Wednesday nights when I start that in February. The teaching series IS canned, sort of, in that I am using another congregation's outlines, but filling in and re-arranging the details to suit our own needs as we begin building the basis for a covenantal congregational membership instead of a "take the preacher's hand" membership. Dever's congregational tools are good work and worthy of re-teaching, but they are not "sermons." They are "teachings." We all gain our teaching tools somewhere, and I'm sharing where I am gaining mine for this series. Can't do more with my schedule and I'm not even sweating it. Church will eat it up. Most solid teaching they've had in decades!