Why my business logic should be moved from simple java classes

I am pretty much inexperienced in design patterns or the technologies like EJB or Spring. I am developing a dummy project on Electronic Banking System. I am using Struts 2 as a framework. Till now I am using simple pattern where flow goes from JSP->action->service->dao where my business logic is in service layer classes that are simple java classes.

Now I want to step ahead. I have seen many people using different technologies for putting their business logic. Some use EJB3, some use spring etc. Now main question is what is the disadvantage of putting my business logic in simple java classes. Like is it a good thing to use ejb or spring or something else for my business logic.