With that handy-dandy spreadsheet someone linked me to, and the wonderful rulebook, I took a look at what the fatigue penalty meant in practical terms. Note that the plate/chain prot values are 20/17, and the enc values are either 8/6 or 10/8, depending upon exact equipment. Note also I'm doing this from memory now, so please double check the numbers.
Working with the 10/8s...
Code:
round 1: no fatigue
round 2: plateboy has -1 def
round 3: plateboy has -2 def, -1 att, -1 crit
chainboy has -1 def, -1 att, -1 crit
round 4: plateboy has -3 def, -1 att, -2 crit
chainboy has -2 def, -1 att, -1 crit
round 5: plateboy has -4 def, -2 att, -2 crit
chainboy has -3 def, -1 att, -2 crit
round 6: plateboy has -5 def, -2 att, -3 crit
chainboy has -4 def, -2 att, -2 crit
round 7: plateboy has -6 def, -3 att, -4 crit
chainboy has -4 def, -2 att, -3 crit
Again, note that plate vs chain is +3 prot, -3 def to start, unless I've missed something else. Note also that spears include repels, which makes combats go longer, which makes life worse for the unfit plateboys.
I don't see how you can justify spending 10 more resources for those units.
-Jeff