Engagement shoots are not only fun nor are they the sole reason for photographers to take in extra cash, but are in fact designed to give the clients a good feel for how they will look on their wedding day, especially if you have never had your photo taken professionally. It's a fantastic time to iron out all the "bugs" ... or ... what "bugs" you in the shots the photographer will take, for your wedding day.
To give you an example, I did an E-shoot once where the clients said to me after viewing the photos, that they wern't all that keen on the kissing shots and requested me to limit those shots to 2 or 3 only on the day. It wasn't that they look terrable in the photos of them kissing, it's just that they realised, that out of all the shots I took, they prefered it if I didn't take all that many of them kissing, that's all (weard as that sounds). Hence I adheared to that request on the wedding day and shot only 3 of them kissing - period!.
It's also a great chance to see if you DID make the right choice for photographer after all ... after all you are investing a lot of money on a photographer and it's better to find out at the E-shoot that the photographer you booked, wasn't right for YOU, rather than after you get your wedding photos back to find that out. It can also serve as confirmation as well, that you DID make the right choice, therefore it's "peace of mind" for you, so you can then look forward to getting your wedding photos back with confidence, without the worries of "hoping" they'll turn out any good hehe.
I'd say, "go for it" if I were you ... what have you got to loose? ... there is NO one saying that you HAVE to purchase photos off the photographer, but "if" they do their job right, then you won't be able to resist purchaseing a photo or two off them and is there anything wrong in that?
It definatly makes for better wedding photos IMHO, because YOU not only find out what you do and don't like in the photos they take, but the photographer also then knows what you do and don't like also ... it's a 2 way street.
It just may after all, be the BEST decision you make for your wedding (right or wrong) ...