I hate the idea of somehow coming up with an equation that "proves" unit "x" is better overall than unit "y". You can certainly perform some rough grouping (elites vs. normal vs. levies), but within the subset of each group, who cares? The real lethality of any unit or mix of units is dependant upon your tactical skills as a general. If your strategy amounts to lining up one group that's slightly better than another and depending upon attrition to win the day......gah!

So go ahead and add some units of theoretically lesser "value" to your armies. If you are a good general, it shouldn't matter one whit. And if you aren't a good general, then practice tactics and styles and methods until you are. THAT is what results in battles you remember and tell stories about. Pah on the slaughterfests!