Yes, you can cancel service if you buy the Gear S3 without contract. The watch works the same except it will not be able to work as a stand alone device for making phone calls and Internet access unless bluetooth connected to a phone or WiFi.
To me, the extra few bucks a month is worth having the watch to be able to work as a stand alone phone when needed. This way I do not always need my huge phone with me all the time. Everyone has different needs, so I can understand some who do not need the added LTE service for the watch. Since you can buy the LTE version for the same price as the non-LTE version, it may be a good idea to have the LTE for a later date if needed. The watch has an embedded e-sim, so you could have service with AT&T or T-Mobile turned on within a matter of minutes. I cannot speak for a Verizon or Sprint version as they have not been released yet. The AT&T and T-Mobile versions are interchangeable for the LTE/4G service.