If you are happy to record the event with a few snaps on your mobile then help yourself. If however you wish to make use of the images at a later date or as part of an extended marketing campaign it makes sense to avail yourself of the official photographer. They are used to the ambient lighting conditions and will produce better shots than average. If you have any specific requests or if you need particular people captured on your stand you can brief them in advance. Much of this information will be in the exhibitor manual also.