About the City Garrison promotion - I think I added an alternative promotion line in v2.6 to modern era which kind of overlaps with city garrison line, this alternative line gives bonuses for attacking as well as defending a city. Edit: Promotion line was Urban Tactics.
I am aware of that. However, Urban Tactics is considerably weaker than City Garrison. First, it requires Combat 1 and either City Raider 1 or City Garrison 1. Since Mech.Inf can't get CG, that means you must waste one promotion on CR1, which has nothing to do with defending. Second, UT gives 10% less bonus per promotion to city defense than CG. The vs. unit category bonuses are nice, but don't stack since each promotion gives it against different units.
So, to summarize, with three promotions (reasonably easy to attain in all cities with a charismatic leader, possible for all with Communication Networks):
CG1-3:
+75% city def, +10% vs. melee
C1, CR1, UT1: +10% str, +10% city def, +15% city atk, +10% vs. gun, wheeled, tracked, clone
-> total
+30% city def against anything you're likely to encounter except siege
C1-3: +30% str
-> this is
better than the UT1 alternative, since it gives
+30% against everything
And for the sake of comparison, some combinations with five promotions (requires considerable effort to get out of the box, even as a charismatic leader):
C1-5:
+50% str, inconsequential heal bonuses
C1, CR1, UT1-3: +45% city def, +10% str, +90% city atk, +10% vs. gun, wheeled, tracked, +15% vs. siege, copter, etc.
-> a bit better than pure combat, at
+65% against most modern era attackers
Plus a few that are not available to Mech.Inf:
CG1-3, C1-2: +75% city def, +20% str, +10% vs melee
-> total
+95% city def against everything
CG1-3, C1, UT1: +85% city def, +10% str, +20% city atk, +10% vs. gun, wheeled, tracked, clone
-> slightly better than pure C+CG at
+105% against most modern era attackers
C1, CG1, UT1-3: +65% city def, +10% str, +75% city atk, +10% vs. gun, wheeled, tracked, +15% vs. siege, copter, etc.
-> worse than pure C+CG, only
+85% against most modern era attackers
Aggressive leaders get Combat 1 for free to Gunpowder units, but those can get City Garrison too. So let's see about one more combo with six promotions:
CG1-3, C1, UT1-2: +100% city def, +10% str, +45% city atk, +10% vs. gun, wheeled, tracked, clone, +15% vs. siege, copter
-> total
+120% city def against most modern era attackers, a bit more against some
Now, how do these number work out with actual units? A Modern Infantry has a base strength of 50 and can get that awesome +120% bonus for a total 110 defensive strength. A Mechanized Infantry has a base strength of 70, but will have to do with only +65% bonus for a total of 115 defensive strength. The latter costs 40% more hammers, for a measly 4.5% increase in strength, and has an extra +2 gpt support cost too.
The conclusion: If you can get the City Garrison line, Urban Tactics is a nice extension to it. If you can get the full Urban Tactics line, it's kinda nice. If you can only get to UT1, don't bother, UT1-2 is so-so. Urban Tactics is awesome for attacking, especially if you can get the full line. Mech.Inf is not a good defensive unit with the available promotions.
Something needs to be done.
Edit: I take it back about it being easy for any leader being to get three promotions for new units from all cities. I was only able to find +5 from civics pre-CommNet, +2 from Pentagon and +2 from Garrison. That makes +9, one short of the 10 required for level 4. Communication Networks enables Supremacy for an additional +3, and I think it's still in the modern era. (Where does the future era begin? I couldn't spot it in science advisor.)