Changing Logos: Why They Do It

Why do magazines change their logos? Logos are their calling card, whether on newsstands or just when the magazine plops on a subscriber's desk or is dropped into their mailbox. Unless the logo is a disaster, should it be changed?

Some magazines like to change their logos. See my earlier postings on Out, Starlog, Fangoria, Männer Aktuell, Fantastic Films, The New Republic, National Lampoon, and Fortune.

Some magazines do not change their logos. Heavy Metal, for example. Or Der Spiegel.
Other magazines have not changed them so much as tweaked them, altering the size or spacing of the letters. Playboy, for example (see photos). Time, for another. Or National Review.

What must go through the minds of a magazine's managers to make them change a logo in which they've invested years and dollars worth of promotion and goodwill? Is the publication flagging? Is newsstand competition heating up? Are they reacting to new developments in the logos of other publications? Are they just trying to make a change for change's sake? Have they lost their minds?
I've already said what I think about the change in Starlog's logo. But I've been through the process twice before on magazines for which I worked, having seen one magazine change its logo from a distinctive design to a fairly boring one. More recently, I had the pleasure on my own magazine of changing a nice-looking logo (that was difficult to make visible on many cover images) to a bolder text that has cleaned up our covers immeasurably.
In the latter case, we had to worry about what our readers would think, but we heard no complaints. So it was a good change. But what, for example goes through the minds of people who change a logo that's been in use for three decades?
"If it ain't broken, don't fix it" is generally a good rule. But when a magazine changes its long-standing logo, it should keep in mind and try to balance its need to be fresh and interesting to its readers (the new ones and the old ones) with its need to be familiar and communicate its quality (and qualities) through its established trademarks.

And, of course, publishers do sometimes make a change – and then change back. Warren's Creepy magazine did it. National Lampoon did it. Will Starlog do it? Should it?
What do you think?