As a Pastor, I very rarely have a guest speaker.
I take 2 Sunday's off per year for vacation (just upped that to 3 for 2018) and I have someone fill in. We have a guest speaker annually for our Homecoming Sunday. I may have a missionary preach on a Sunday night once or twice per year. We usually get an HAC tour group on a weekday and let the tour preacher have the pulpit, but on the rare occasion that they come on a Sunday Morning, I still preach.
Perhaps the main reason is that I feel obligated to deliver 3 messages per week to my church. Since they pay me, I feel bad taking money and not preaching to them.
I don't "guard" the pulpit out of pride. I encourage them to listen to other preachers. I purposefully don't teach Sunday School to ensure that they are influenced by other members, not just me. I never tell our Sunday School teachers what to teach nor do I sit in their classes to make sure they "stay in line."
Still....I am very protective of our Church's message and culture. I think Dr. Hyles was a well.