@MarshMadnessif you have the enterprise subscription on your MX then you may well want to leave another more fully featured device on the WAN side of it, however it is fairly unlikely that the AT&T device is such a thing, if you let us know the model I'm sure someone here will know. If you have the advanced license then you definitely don't need the provider's equipment unless it is a media convertor (DSL to eithernet etc.) or the contract stipulates that you must use their edge device.
You can tell what MX license level your organisation has by going to Organization / License info as below, as you can see we only use Enterprise as we don't use our MXs as corporate edge firewalls:
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