I might need help from someone with phrasing the idea, as people seem unable to understand something that is pretty obvious in my eyes.
1 - This system is like any other spoils system. You can "naturally" collect up to 5 spoils. You are forced to cash everytime you have 5 or more spoils. The only possibility of holding more than 5 cards at once is, like in any other game type, killing someone else and having a midcash. When I said in the OP that we could cap that at 5 cards or go further, allowing for more powerful midcashes, I thought I had established pretty clearly that the only way to hold more than 5 cards is through an elimination.
2 - Another misconception is that you'll get armies based on the number of spoils you trade. No. You always trade 3 spoils (like every other game type). You will, however, get armies based on the number of spoils you hold when you decide to cash (and only 3 will be cashed). I thought the "trade 3 spoils when you have 4/5/6/7/8/9" had made that clear, but apparently not.
I'll try to make an example on how the game would look like (assuming you take a territory everytime):
round 1 - you end with 1 spoil
round 2 - you end with 2 spoils
round 3 - you end with 3 spoils
round 4 - you can cash 3 spoils for 4 armies (and end round 4 with 1 spoil) or not cash and end with 4 spoils (I'll assume you did not cash)
round 5 - you can cash 3 spoils for 6 armies (and end round 5 with 2 spoils) or not cash and end with 5 spoils (I'll assume you did not cash)
round 6 - you are forced to cash 3 spoils for 8 armies (and end round 6 with 3 spoils)
round 7 - you can cash 3 spoils for 4 armies (and end round 7 with 1 spoil) or not cash and end with 4 spoils (I'll assume you did not cash)
round 8 - I'll assume you can kill someone without cashing your spoils. The person you are going to kill has 5 spoils. You kill that person and thus hold 9 spoils. You can then:
a) capped version - forced to cash (from 9 to 6) for 8 armies, then forced to cash (from 6 to 3) for 8 armies and then cash or not (from 3 to 0) for 4 armies -> you'll get between 16 and 20 armies
b) uncapped version - forced to cash (from 9 to 6) for 20 armies, then forced to cash (from 6 to 3) for 10 armies and then cash or not (from 3 to 0) for 4 armies -> you'll get between 30 and 34 armies
You see, the game type and number of armies usually gotten resembles "flat rate" system, but the ammount of armies you get are not determined by luck, they are determined by your decision on when to cash. Accepting the "uncapped" version of the suggestion, we could add a small "escalating" factor to the idea, in which killing someone with several cards would allow you to make a pretty powerful midcash and possibly sweep the board.
I hope my idea got cleared now. I'll be accepting ideas on how to make the OP clearer!