I suppose you're right about the accordian issue...it does make sense...but there can never be too much AL-cordian on an ALbum in my Al-pinion
I think what happened, was after the debut album, Al realized he could be very successful, but he also decided that he needed to make his parodies sound as close to the originals as possible for added affect. He probably dropped the accordian from the originals because he wanted to be able to hit more GENDERS of music that don't include accordians (hence the regge, B52s, and rock songs on In 3-D).
Then, he probably decided to include a polka medley on each album in order to be able to use his accordian on at least one song per album. When Even Worse came around, and he used accordian on Lasagna, he felt he didn't need a polka because he already had an accordian song.
It wasn't until the Internet and the Midnight Star came around when Al realized that his fans actually really enjoyed his accordian fuled songs of his early days. It was then (mid-nineties), that we started to see songs with accordian music, other than the polkas. He gave us accordian on Headline News, EYKIW, Syndicated Inc., and MBILWEV.