I charge at the enemy with that set-up. The legionaires are just to absorb arrows/knight charges for the first 2 rounds of battle really.
Why not use cold or heat? Because each also causes a 5% income loss(although I took DrP's scales+some cold so I could buy growth).
edit: You can just edit your post to add small notes like that thing about drain
.
Yes, drain 2 would give +1 MR but your vestals already have enough and you can boost your low MR undead with spells. That boost also works for both sides of the battlefield, and with your magic being MR negates(nether spells, shadow blast and such) you want the enemy with as low MR as possible.