The justification only needs to be a couple of sentences.
The easiest thing to do is to state the problem that existed
in RFC1759 and why the change solves the problem. For 
example what Bob wrote as justification for the two 
counters added to the General Printer Group is fine:
  The objects prtAlertCriticalEvents and prtAlertAllEvents have been 
  added to allow management application to easily determine if there 
  has been a change to the alert table. Without these counters, the 
  entire alert table must be read and parsed to determine if the state 
  of the printer has changed. By using these objects it can quickly be 
  determined whether or not there has been any change since these objects 
  were last read, and which class, critical or not, has changed.
1. Several Textual Conventions were added in the new
   Printer MIB that were not present in RFC1759:
   z Tom Hastings write up why new TC's were added
General Printer Group changes:
1. Six new objects were added to the General Printer Group:
  prtGeneralStartupPage         PresentOnOff  - Tom Hastings
  prtGeneralBannerPage          PresentOnOff  - Tom Hastings
  prtGeneralAdminName           Octet string  - Jay Martin
  prtGeneralSerialNumber        Octet string  - Harry Lewis
  prtAlertCriticalEvents        Counter32     - Bob Pentecost
  prtAlertAllEvents             Counter32     - Bob Pentecost
Input Group Changes
1. A group was added: The Input Switching Group
  prtInputMediaLoadTimeout      Integer32     - Angelo Caruso
  prtInputNextIndex             Integer32     - Angelo Caruso
Print Job Delivery Channel Group changes
1. One new object was added:
  prtChannelInformation         DisplayString - David Kellerman
Lloyd Young                       Phone: (606) 232-5150
Lexmark International Inc.        Fax: (606) 232-6740
740 New Circle Road NW            internet: lpyoung@lexmark.com
Lexington, KY 40511