I thought it was probably deliberate, in so much as it is consistantly missing from both the main body of the rules and from the charts of HP costs to improve wizardly abilities (by which I mean they are not given a "concentrated only" cost). Of course the cost of improving as a adept if you haven't concentrated is pretty horrendous (but noone in Glorantha knows the numbers) and many schools/orders etc may insist on concentration anyway.
I suspect it is not strictly "necessary", because where a Theist or Spiritist is dealing with something with some level of intelligence, a Wizard is dealing with impersonal essences which don't know whether he is dabbling in forbidden methods...
I also suspect that since there is less scope in regular church blessings for a user of essence magic to gain "personal" magic outside of common magic, there is more reason for such people to continue to use common magic than animists (who have a wider choice of spirits) or theists (who can interpret their affinities and feats more widely)
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