magicJack Go is basically a simple VoIP Analog Telephone Adapter (ATA), locked to the magicJack service provider, which comes with a telephone number and a year's worth of service. It has to connect to the Internet, just like any other ATA. OBi 20x devices are more powerful and complex ATAs, which can be provisioned (set up) with up to 4 service providers. They also connect to the Internet.
You can't operate a magicJack Go without an internet connection. Connecting it via OBiLINE adapter alone isn't going to make it work; the OBiLINE is a RJ-11 telephone line interface, not a network connection. All the OBiLINE would do would be to allow you to access the OBi's calling services via the MJ, or vice-versa.
Whether or not you connect the two devices is up to you, and simply depends on whether you value the MJ enough to keep it in service. Aside from whatever sunk cost you have on the MJ and its remaining months of service, there is no advantage to keeping it vs. just using an OBi.
