The behavior of this spell implies something easily on par with Strings of the Unwilling Marionette, yet it uses a Base 4 (5 once you include the fine movement control) instead of 10. What am I missing here?
Although it isn't specified in the spell's description, I'd say that it only controls the target if he lets himself be controlled. If the target resists, activities that require fine control cannot be achieved by either the target nor the caster. The spell also wouldn't control the target's ability to speak.
But as you say, the lower base shouldn't allow the same level of control. This should have been included in the spell's description IMHO.
It's in MoH, so expect buggy-ness. Sorry. 
In MoH's defense, many guidelines are open to interpretation. So it isn't surprising to see some spell designed with more lenient interpretations, while others may be stricter.
Storyguides and players should always check to make sure that they agree with the specific interpretation used in a spell, as well as discuss what each spell's limits would be in a specific saga. Even those in the main core book can vary wildly from one saga to another.