This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
The only time I'd recommend using the same form bean for multiple actions is if you're building a "wizard" where the user enters information through multiple pages and does a "save" operation when finished.
One big disadvantage to using a single form bean for multiple actions is that it makes it much more difficult to validate data, either with your own code or using the Validation Framework. Besides that, there's just the general awkwardness of having a lot of extra properties in the form that you don't need for the current page.