Hosted apps like Yahoo Store do not allow complete customization of all pages and interfacing with other business systems. For example, with openacs (and ACS before it) you can interface with computer controlled manufacturing equipment, so no human intervention is needed between order and manufacuturing products (ArsDigita did this with a devision of Levi's). One can integrate ecommerce with classes/events and charge users that attend, once paid one can see XYZ on the website, etc. All without human intervention. But these are large site services and not small ones. For small services ecommerce can definitely be useful as well but you need to decide wheher or not the hassels of having a specific to your needs custom solution outweigh the tradeoffs of a not so custom solution.
