mitsho
Deity
The reason behind the plague is to speed up the game by reducing units and thus reducing computing time. It does occur evenly spread out on the turns (that's why ;-)).
Regarding the strength (it does kill the great majority of your units and some cities have no units in there afterwards), this is debateable. However, at the moment it is balanced and there are some techniques of avoiding it (having a 10-health point difference (if you have 11 unhealthy points in one city, you need to have 21 or more); whipping down all your cities to that level; closing open borders; ....)
Of course, the feature is not really liked because it is absolutely a negative feature which punishes you. But if you are prepared, you can use it against your enemies ;-)
Regarding the strength (it does kill the great majority of your units and some cities have no units in there afterwards), this is debateable. However, at the moment it is balanced and there are some techniques of avoiding it (having a 10-health point difference (if you have 11 unhealthy points in one city, you need to have 21 or more); whipping down all your cities to that level; closing open borders; ....)
Of course, the feature is not really liked because it is absolutely a negative feature which punishes you. But if you are prepared, you can use it against your enemies ;-)