Didn't add it for my first iPad 3, didn't know what I missed and used it mostly indoors as it was when iPads were hella heavy, no loss.
Then cheaped out when getting the 10.5" Pro, turned out I travelled a lot during the ownership and quite regret not having SIM, had to deal with spotty public WiFi all the time overseas, and iPhone tethering absolutely sucks.
With the M1 12.9" I made sure to add it since the iPad together with the KB and Pencil is a sort of large investment, so may as well. Then pandemic came... used the SIM slot a grant total of once. Was supposed to travel more recently but I found out how good the portable modems have become, so ended up not needing the iPad's internal SIM at all.
If I were to get an iPad Pro M4 right now I would save the 200 bucks.