From my opinion, a wedding photographer should offer this as a default service since it not only benefits the bride, but it also benefits the photographer.
An engagement session allows the photographer to see how best the bride (and groom) can be photographed in what light. It also gives the photographer the test venue to see how the couple can look good together and how to capture their personalities as individuals and as a couple. It helps build that comfort level as well between the photographer and the couple - something that would be vital on the actual wedding day. It also allows the photographer to "train" the couple on how to look best on camera.
As the bride, this would most likely be the first time you would also be working with the photographer so it serves as a good venue to start the comfort level and trust building - vital vital to have those natural looking photos on the big day. This is also a time that you can see the vision and style of the photographer and tweak expectations if need be. This is also a good time to make requests - eg can you shoot from my left side - although quite frankly - your photographer should know how to make you look the best way possible. As a bonus - you can work with your photographer how to use the output photos for your "save the date" cards, invites, facebook, announcements and even maybe get an engagement album in place.