I took a crack at some math relating to the rune mechanic and figured that I would share some information. The first chart below shows every combination available from the 5 runes cast at the start of each turn, and the percentage chance of getting that exact combination. Using the table below, you can calculate the likelihood that you will have an appropriate "mana curve" for the abilities you take and can determinate the exact percentage you will have of getting what you need each round.
The second table shows you the min / max of each rune, and the percentage chance you will get a specific number, with the percentage chance you will get "at least" a specific number.
The TLDR version is that there is a 62.5% chance each round that you will get one of each color of runes. That probably means that most builds will have a way to "dump" each color of runes so the army isn't wasting resources.
Just wanted to share.