Decrypting your entire army is a VERY good plan, logistically speaking. No upkeep, all the same abilities. (This has been untested on casters, though.) The drawback is the fanatic loyalty to Wanda, which Stanley and Parson are not entirely comfortable with. And yes, since you have a croakamancer on your side, you might not want to kill&decrypt units who could fight, get killed, and then be decrypted for a whole new lifespan... but the whole having-no-upkeep thing might make that worth it. Heck, if you save the upkeep of 1 unit for 3 turns, wouldn't that be a good investment to take that savings and pop another unit?
The majority of GK's armies are decrypted at this point. It's a large contributing factor to why they can expand so rapidly without worrying significantly about expanding faster than their resources allow for with a wide city base - very little resource demand, comparatively.
I wouldn't read too much in the Ossomer turning. It's a big deal, but I would not assume that any mook can jump ship once Wanda looks away or wanders to a different hex. Ossomer was a high level, royal, warlord, heir. Who had plenty of good reasons to turn. As has been stated, Ansom was soundly defeated and convinced of the rightness of his loss, titanic mandate, holy tools, etc. Ossomer didn't have that conviction behind him, so he was not as fanatic.