Shoulder Rigs are one of those pieces of equipment that are very polarizing in a specific field, there are people that swear up and down that they are great an then there are people that prefer other things instead.
There are several reasons that you would want a shoulder rig for your DSLR Video. The first of which is perception, people assume you are talking stills when they see a DSLR, as such they treat you like you are talking stills. If you have your camera in this shoulder rig is it very obvious that video is your purpose and not stills.
Other arguments included stability when shooting things like b-roll or run an gun shooting where you need to be quick and mobile. In long run though the benefits of a shoulder rig will vary greatly depending on your style of shooting. If you need to be mobile and prefer not to have your camera locked down on a tripod or mono pod then a shoulder rig is a great alternative.