I guess there was a good reason why the game initially didn't allow the recruitment of additional garrison forces. It appears the AI likes to recruit such units to its detriment:
The above is the garrison of Moscow in 1712 or 1713. Almost all those units are immobile garrison units.
As the Swedes, I was wondering where the powerful AI armies disappeared to after a certain time. All I was seeing were penny-packet armies. The AI going recruitment crazy with relatively useless garrison forces may be a factor why its field armies shriveled-up.
Edit: I took a closer look at that Moscow garrison and this is what it consisted of:
9 units of garrison infantry including one each from Denmark and Saxony
16 units of fort guns
2 units of coastal batteries
2 supply depot units
That is an awful lot of resources put into a lot of rather worthless units. It appears the AI needs some help as to its recruitment logic and/or the allowing of recruiting additional garrison units should be reworked or rethought.