it's the extra space for the sound. Basically, if you're doing Dual Layer, you should be able to stick a fair amount of extras, get rid of some dot crawl and artificating and chroma noise, and that'll leave enough room for the PCM track along with a 2.0 track. That would be the major benefit of an LD rip on Dual Layer. Improved picture shouldn't be the main aim for a Laserdisc rip--as has been shown with the anamorphic set, you're not going to improve on it enough to where just picture quality is a main issue.
Sound quality, on the other hand, should dramatically improve. If you're good, you should be able to matrix out a 5.1 soundtrack from that PCM track and get it sounding better than the regular 2.0 surround, and still have room for slightly improved anamorphic video.
The aim here seems off. Don't abandon Dual Layer transfers just because you can't get the pic lightyears better than LD.