My main reason for dual booting was, while I preferred pdaXrom (I liked the more "pure" linux environment) there was no good PIM app that would easily sync with my work calendar. I do need the PIM for work even though most of use of my Z is "play" (mp3 player, video player, video games, linux learning lab and war walking) Once KOPI & KAPI was compiled for pdaXrom, I stopped using Cacko & stopped dual booting.
Performance-wise, it was slow due to the limitation of SD speed. But it worked, the only problem was the miscalibration of the screen, which wasn't correctable. It was cumbersome & slow to have to reboot to move from either ROM. The benefit was the wide range of apps available. In terms of practicality, X/Qt seems a better way to get the benefits of having an X environment easily accessible from within Sharp-based ROMS. I tried X/Qt and liked the ability to to move quickly into it without rebooting. Down-side, fewer apps available. However, Dual booting might be a way for someone to try out the pdaXrom without having to reflash (and repartition) their Z.
The other benefit to dual booting for me was gaining knowledge about how linux works, but that has been the story of my Z usage. I was a linux noob when I first got my Z in Sept 2003. Now I'm fairly proficient have all my PC's dual booting into linux. I started out with Mandrake, then moved to Slackware and currently have settled on Gentoo as my fav distro.
My thoughts, good luck and have fun