When you define a server in server view, then it will create you a server runtime library with server libs (including servlet api), that can be assigned to your project. However, then everybody that uses your project, need to create the same type of runtime in his/her eclipse workspace even for compiling.
If you directly download the servlet api jar, than it could lead to problems, since it will be included into the artifacts of your projects, but will be also present in servlet container.
In Maven it is much nicer, since you can define the servlet api interfaces as a "provided" dependency, that means it is present in the "to be production" environment.
