While most other infinite spells see at least some use. Necromancy seems to find little use for the Ray of Enfeeblement infinite casting.
My suggestion would be to switch this spell to Frigid Darkness. As this would give Necromancy spell focus more of a use besides a buff for undead summons.
If there are still balance concerns around an infinite cast version of the spell my second choice would likely be negative energy ray, as it could still have some utility in supporting your undead summons.