I actually think mnemonic devices are an application of critical thinking to the task of rote memorization. Take the knuckle method of remembering the Gregorian calendar, for instance - where you simply count off the months starting with your leftmost knuckle going right, and all the months on knuckles represent 31-day months. That's not just memorizing the months; you're taking a specific piece of information, a pattern, about the months and creatively applying it to something completely unrelated so you can learn the months more efficiently. Mnemonics are effective, I think, specifically *because* they go beyond rote memorization and employ your critical thinking capacities. I can say at least for myself that that's usually the case for me.
Now, true, just regurgitating mnemonic devices without considering them is just a more efficient means of memorization. Hopefully, though, the revelation of mnemonic devices to a student can spark that student's creative thinking and lead the student to find his or her own... and even if the student fails in that attempt, merely attempting to do so requires critical thinking, which is already an improvement and a worthy means in and of itself.
So fire away with your Roy G. Bivs and your Thirty Days Hath Septembers, I say, because in creating them and in considering them you're employing your brain more constructively than you would be just memorizing colors and months without them.