Brian,

 This is fact of physics. Parallel walls, whether they be in a room concert hall, or speaker cabinet, will excite frequencies related to the distance between the walls. What you don't always have the option of is seeing what is inside of a speaker cabinet. I made a set of speakers that were anything BUT parallel inside, but from the outside, looked liked boxes. Also, to some degree, using damping material reduces the effect of parallel walls.

Having considered the idea that a speaker box can, and does have resonance, may not be all bad. In some earlier models made by companies such as JBL, and ALtec, the sound of the cabinet became part of the voicing of the speaker. This was not just for reasons of bass enhancement, but higher frequencies at which the cabinet walls themselves contributed. I mean, just as a wooden instrument has it's own tone, due it's body, so did the cabinets of a few speakers made way back then. I don't think that we can put all box speakers into the same category and claim to be able to hear resonances of the same level. One cabinet will produce different sound (or distortion) quite differently than another. If a cabinet is designed properly for it's purposes, and resonances/dropouts are dealt with, then we should be able to hear little, if any of those troubles while listening to music. No doubt, there are a great number of speaker boxes out there that have little more than room for the drivers and crossover, without any consideration for much else that might need attention.
Let me put it another way. When we listen to speaker inside of a room, isn't that room just a very large cabinet that the speaker(and you ) sit in? One is room acoustics, the other cabinet acoustics.

Brian,

 You are right on track. Unwanted resonances cause problems that make music sound, well, not like it should. It is tough enough to fool the brain into accepting a great performance as sounding correct by way of the medium in which it was made. Recordings, and the place that it was recorded can be less than desired. By the time we reproduce that recording into a less than perfect room, the chances of the illusion being successful drop. Now, back at the speaker cabinet, the goal is to hear the driver(or to have it disappear completely), and nothing else. Not cabinet resonance, not reflections from the front and sides of the cabinet. I have been very fortunate to be able to control most of these issues in my listening room, but it took time and effort.