It can make combat pretty complicated for each of your opponents, and on the plus side, it can also help you to push damage through
Keiga, the Tide Star was a powerhouse in Standard, and Ryusei, the Falling Star, Yosei, the Morning Star and even Jugan, the Rising Star were good
You strip your opponents hand with cards like Thoughtseize and Hymn to Tourach, then keep the heat on with threats that grow or generate value over time
The sieges flip condition is removing defense counters via combat or direct damage spells
An example of this is the Nemesis cycle of Laccolith cards in red that all have the same ability
If I had to venture a guess, Id say most decks should run no more than 20 but thats a ballpark figure