I know with AT&T at least, the modem/router is provided as part of the monthly cost. They do not bill the unit separately, so it is basically free.
You can go with your own router with an optical input, but I haven't seen much need to do so in this case. Pros would be better security since the provided units will be the most likely to be compromised. Con would be the initial setup.
When I had comcast they did bill separately for the modem, so I bought my own netgear DOCSIS 3.0 unit and saved $8 a month, which paid for itself before the introductory price was up. With comcast they don't properly provision non-standard units, bandwidth increases I always had to call in for. They expected people to use their model, so would only send update signals that effected them.