variable naming convention question - from 1.7 General Style and Syntax guide

[eluser]skattabrain[/eluser]
First of all ... /CodeIgniter/user_guide/general/styleguide.html is a great addition to the documentation. But I have a question ...

Variable Names

The guidelines for variable naming is very similar to that used for class methods. Namely, variables should contain only lowercase letters, use underscore separators, and be reasonably named to indicate their purpose and contents. Very short, non-word variables should only be used as iterators in for() loops.

Is using a single letter for things outside loops bad for the reason of not being obvious as to what the variable is? Or is there another reason?