Be it a lawyer, investigator, or just an IDI theorizer...I myself don't recall hearing explanations about the pineapple.
I lean towards an intruder, always have, but I could be convinced it was one or both Ramseys (I don't think it could have been Burke for various reasons).
I've never seen the pineapple as a significant issue for several reasons.
First, the speed at which foods move through the digestive tract is highly variable. Solid foods move slower, high fibre foods (like pineapple) move slower. Medications can affect the speed--does anyone know if JonBenet were being given any sort of medication for the bedwetting problem? If she was, that would slow down her digestive system. Unusual activity, incipient viral infection, even just the consumption of cold drinks, there are many factors that can slow down the digestive tract.
Over the years many coroners have made guesses about the length of time food found in the digestive tract was there, only to be proven factually wrong, sometimes by a factor of ten.
In a healthy human being, just emptying the stomach can take anywhere from fifteen minutes to ten hours. So gastric emptying is really pretty useless to use as an indication of the passing of time.
The state of digestion of foods found in the stomach or upper small intestines can vary widely in appearance. My father (a surgeon) once operated on a man who had a faulty pyloric valve (valve between the stomach and small intestines) who had eaten a couple slices of orange ten days previously that did not move out of the stomach; he said that the pieces of orange looked like they had been freshly eaten. Other surgeons have reported similar episodes. Why didn't the oranges decompose? Apparently the stomach was acid enough to preserve them, in the same way cucumbers are preserved as pickles by vinegar.
While no one remembers giving JonBenet at the White's party, it sounds to me like there were a bunch of people and kids present. Two kids could have shared some pineapple without anyone noticing, so long as they weren't running around and throwing it at people's hair. Or one kid could have given JonBenet the pineapple.
Another possibility--the creepiest one--is that if the murderer was an intruder who knew the family well, he could have given JonBenet some drugged pineapple at the party, knowing she liked it and would eat it. The Ramseys did say she fell asleep on the way home and remained asleep while being prepared for bed.
Another possibility is that JonBenet did get the pineapple at home. Frankly, I think Patsy was an airhead about domestic matters. It was the holiday season, there was a lot of stuff to do and I'm unconvinced that she really knew for sure what she did or did not have in her pantry. Could there have been a can in there that she didn't remember? I think so.
Was her memory that good? People usually think they remember stuff but, for example, look at all the people who try to lose weight and start keeping a food journal. They discover that they were eating far more food than they really remembered (which explains why they wanted to lose weight).
Look at Dr Elizabeth Loftus's research on memory, particular memory as it concerns routine events. While it seems like someone would recall every detail of the day before their child died, Dr Loftus's research indicates otherwise.
And finally, in every single criminal case I have ever read about, there are elements that were thought to be relevant to the crime that turned out not to be. Prosecutors go to court with cases that do not include an explanation for every element found at the crime scene because such an explanation usually is not possible.
For example--FBI agent, shot by hit man in motel room. Knocked unconscious by the shots, comes to some time later, gets up out of bed, goes into the bathroom and then goes into the living room area of the suite. Does he think to call for help? Nope--he was discovered by his co-workers when he didn't show up for breakfast. Why did he go into the bathroom and living room? He remembered doing so, he remembered turning off the TV but he doesn't remember any reason to do those things.
So, the pineapple may indeed be a key piece of evidence. Or it may be a red herring.