The thing about combat GGs is that I usually siege a city until my best unit is no less than 95% chance of winning. Most of the time, even a mediocre unit will have a greater than 99% chance of winning... so I really don't see the point of the combat GG. If I am going to war enough, I'd rather use it for XP in my military city
A powerful defender or two (such as a CG longbow) is likely to chew through a lot of siege, even as a combat GG such as a CR III Combat III mace could kill it well over 95% of the time. This saves you units and WW.
With theocracy or vassalage you get 2 promos. It takes a lot of them to get non-CHA to 3 promos/turn for non-mounted troops.
Ever actually had a combat VI march knight? You get 2-3 of them and all of a sudden you stop losing more than maybe 1 siege unit/city, if that.
I had a domination win on emperor with 2 c VI march elephants, a super medic, and a CR III C III sword. The rest of the stack was stock elephants, catapults and axes. The opponent I was fighting had longbows at the start and very shortly had maces.
After suicide of 1-2 CATS (not trebs) into longbows I was able to take them down - the only way this was possible was my rifle-strength elephants who usually had full HP the turn after they attacked. Compared to having some extra combat III elephants, IMO this made it go far more smoothly (both elephant GGs had over 70 xp, the sword had over 50 IIRC, so there's an idea of their utility). How many cats did I save? Who knows. At least 10 for sure, possibly closer to 20. Would having a 3 promo unit out of the gate helped more? Doubtful.
A CR III C III sword is almost a CR III praetorian. When it is upgraded for free into a mace later, it owns pretty much all defenders for a long time with even a tiny bit of collateral (as in, a normal CR III mace would beat an unpromoted rifle in a city, so imagine what a C III CR III would do against anything before that, especially with a splash of collateral).
Settling can be useful too of course, but IMO people ignore combat attaching too much.