Lots of good points being made here.
I agree with Metatron and others, that the change required fewer mags to be printed, thus cutting operating costs.
I also agree with Heath N, who said "The public WT is to GET THEM IN......the study WT is to KEEP THEM IN."
Watchtower claimes to speak the "pure language" of truth (Zeph 3:9), but in reality has always spoken two languages. The first language is to attract people, and is usually based on a Bible verse that promises a paradise earth, the removal of wickedness, a world with no hunger, an end to all suffering, etc. This is fairly effective at getting people's interest since virtually all people would love to have these things in their lives.
The second language is Organizational. It is the language of Watchtower, the language that keeps people locked in. It is full of man made rules that only pretend to be supported by the Bible. I think of it as the language of the cult, since many cults use a similar technique of luring people in, then keeping them trapped by promoting the cult's teachings, undersstandings, and loaded language.
The transition from the first (soft) language to the second (hard) language is subtle and clever, so that a person may not even notice that he has been moved away from the 'truth' of God's Word to the controlling language and unique understandings of the religion (cult).
I believe Processor illustrated it very well,
Brochure: "God will do away with the wicked."
Public Watchtower: "God will do away with all people who do not obey him."
Internal Watchtower: "God will do away with all people who do not obey the Governing Body."
Watchtower took advantage of at least two benefits when it cut magazine production. 1) Costs were cut, and 2) by having a Public edition and a Study edition, promoting Organizational interests by the use of the double language became easier.