Drew, your last point about inconsistent (or even contradictory) rules in WT literature is well-taken. Some JWs have a talent for finding WT articles that justify whatever they want to do. For example, in the past I knew some fun-loving JWs who were constantly being accused of "stumbling" other JWs. These fun-loving Witnesses found some statements in WT literature that a strong Christian can't (or shouldn't) be stumbled--in other words, if you got stumbled it was your own fault.
Situations like that are what lead to "liberal" or "conservative" congregations, circuits, etc. The WTS writes its literature in such wishy-washy fashion, trying to create the illusion that we're all mature Christians and they aren't making rules. Most JWs see right through it and realize that when the WTS says things like "most mature Christians would not do XYZ" and understand that the WTS is saying not to do XYZ. But others (like myself in a previous life) would see this as a loophole and say "well, maybe most wouldn't do it...but they're not saying I can't." That's where elders come into play.
If a body of elders interprets WTS directives in a strict way, that body can impose its interpretation on the congregation. This has its most immediate impact on servants/elders/pioneers because the body gets to decide whether they are "exemplary." And this "exemplary-ness" is based on the opinion of the body of elders. Period. But this can also have an impact on the rank & file.
For example, the WTS has had general directions regarding parties and weddings for years. The WTS has often cautioned against having large parties. Some bodies of elders understand this to be a rule that JW parties must be small (no more than X number of attendees). If a body decides this, then if someone wants to throw a big party, the elders and their families would discourage it--and at the very least refuse to attend. This would have a chilling effect on the congregation. Most good JWs wouldn't want to attend a party that is disapproved by the elders, and most JWs wouldn't want to throw a party that's disapproved by the elders. Thus, the result of these elders' interpretation of WTS rules would be no more parties larger than a certain size.