Strands of Astral Power. Worth it?

[Dedas]

Astral mages have extra MR due to astral you know so it is no wonder the spell doesn't work.

As for +1-3 difference in MR matters? Well not if you have 20 MR against the base check against 10. But a standard mage with 13-15 MR in your dominion with magic 2 (for example), that takes it down to 12 or 13 is a big difference - just look at the table in the manual for the chances of penetration.