I do agree, it have been nice if it was part of the action palette. The original PSO on the Dreamcast was pretty much the same way. A, B, and X could be customized and Y was used to access the visual keyboard, word select, and symbol chat. PSO ver2 introduced a "quick menu" you could access by pressing Y when holding R. This quick menu allowed you to quite easily change weapons, use items, and cast techs not in your action palette. A couple easy to hit buttons on the right side of the touch screen would have been nice. Oh wells.
PS0 does have some shortcuts. L + Start will take you to the Item menu while R + Start will take you to the Equip menu.