Oh yeah, Recettear is half sim/half dungeon crawl and its a pretty decent little game.

But yeah, thinking more about it, genre shift is a nice way to pace a game and build anticipation for something. It helps when the theme ties the two genres together tightly.
Most RPGs switch back and forth between two sections that are very different - one where you're battling and one where you're buying/selling items and conversing with townsfolk. Aside from giving you a chance to take a break, each section gives you ways of preparing for the other section. In towns you buy items to prepare for battle and in battle you earn gold to buy more items with. It's a positive feedback loop that feels quite nice if both parts are done well.
(
EDIT: My bad, I didn't see the part where Azure said the same thing.)
Plus having different aspects to the game can make it feel richer.