TLDR:
1. Strong foreign language skills from overseas missions.
2. It's easier getting them security clearances since they don't use drugs or alcohol.
replies(6):
From the members I have conversed with, they are forbidden from using caffeine.
1. Unclear doctrinal specifics
2. An orthopraxic rather than an orthodoxic culture
The first item is that the only doctrine possibly relating to caffeine is a single verse in modern scripture that says "hot drinks are not for the body or belly". Long-standing teaching by leadership has merely narrowed down "hot drinks" to mean tea and coffee.
The second item is by far more important in the debate: Mormons are _highly_ orthopraxic, meaning that you're usually free to hold heterodox doctrinal beliefs as long as your public life and behavior reflects the common orthopraxy. Or, to put it simply, the public appearance of righteousness is culturally far more important than internal doctrinal beliefs (this is, ironically enough, not technically doctrinal). The same chapter that defines "hot drinks" (coffee/tea) as not good for the belly defines beer ("barley... for mild drinks") as entirely appropriate, but since the orthopraxic behavior is to be seen as avoiding coffee, tea, hard liquor (which is specifically called out in the same scripture) then avoiding anything above and beyond those is often seen as an increased sign of righteousness.
So you'll often have arguments between Mormons who follow the letter of the law and others who follow what they define as the spirit of the law. And since coffee and tea both contain caffeine then many Mormons will avoid caffeine as well.
You'll find this same argument about following just the doctrine defined in the open canon versus following behavioral practices above and beyond it in other aspects of Mormon life, such as: not calling members of the church or the church "Mormon," payment of 10% of monthly income (though the scriptures call for 10% of an annual "increase"), women only wearing at most one set of earrings, no dating for youth below the age of 16, men applying to serve missions the instant their 18th birthday arrives (though the window for honorable service is many years wide), no clapping in meetings, no drums or brass in meetings, and so on for many other cultural practices.
This is, as you can probably recognize from some of the items in that list, in no way a phenomenon isolated to the LDS religion, but it does inform the inevitable debate you'll hear if you ever bring up caffeine in a group of Mormons.
> Orthopraxic v. orthodoxic: In the study of religion, orthopraxy is correct conduct, both ethical and liturgical, as opposed to faith or grace.[1][2][3] Orthopraxy is in contrast with orthodoxy, which emphasizes correct belief. The word is a neoclassical compound—ὀρθοπραξία (orthopraxia) meaning 'right practice'. [1]