The thing is, you don't absolutely have to cast two, or even one a turn if tactics call for it. The mana return you get from the spawn points is still there. If you can't shape the game to keep yourself protected, (didn't put on the cloak, armor, set up a wall, use your zombies to attack inbound enemies, etc) the spawn points allow freedom of movement until you get forced to commit to an action and will likely still be there next turn with more mana on them. Take a breather and cast drain life on something attacking you, wall yourself off, or whatever you think will stop the current issue.
As far as zombies being bloodthirsty pests, while that's true, that is baked into the mana cost and ability set. If you want capable guards, you play as the priest/ess or warlords. Forcing a Mage to fight or counter most of the creatures who come his way is the calling card of forcemasters as well, so I've always just had zombies engage, usually under a frenzy, whatever comes my way.
Doesn't always work, but is about as reliable as most builds.