Strands of Astral Power. Worth it?

[Tuidjy]

Jazzepi, the enemy mages in my dominion range from MR 18 to 26, with the majority
at MR 20. The difference between Drain 2 and Magic 3 is 3MR. That would give
a bonus of chance of 8% (Wowsers!) for the wimpiest targets, and no appreciable
bonus for the tough ones.

Mine is a simple argument. If the spell depends on the modified magic resistance
of the target, it should depend on the modified penetration of the caster.