[chainingsolid]

Common no, very easy to proliferate though as people become aware of the savings possible. And the 2 cases I've seen where litteraly order the same model online and swap it, no configuring required. And it wasn't even the family tech support guy(me) who came up with the idea. The ISPs incuding the router as a monthly line item on the bill are litteraly indirectly asking you to do this.

[SoftTalker]

Comcast/Xfinity in fact gives me a discount for using their router. Probably because (a) it lowers their support burden and (b) they are logging and selling my web traffic or at least DNS lookups.

[remram]

That's surprising to me, it was when I used Comcast (2016) that I first purchased a cable modem. It did save me money.

[SoftTalker]

Oh I also forgot that connection sharing thing they do where they broadcast a second SSID called "Xfinity WiFi" or something like that so that anyone with an Comcast login can use your connection.