What's the purpose of rationalization, specifically in modeling human behaviors?
It is true that people use rationalization to prevent/avoid/justify disappointment?
I've commented to request clarification of your first question, but would like to edit this answer to address it too if you can clarify for us.
As for your second question, I'll begin where I often do when researching simple questions: with Wikipedia. Within our psychological context, it defines rationalization as a defense mechanism (citing Schimelpfening, 2011). By definition, defense mechanisms exist "to manipulate, deny, or distort reality in order to defend against feelings of anxiety and unacceptable impulses to maintain one's self schema." (I found the cited link broken here, FWIW.) Further, "When anxiety becomes overwhelming, it is the ego's place to protect the person by employing defence mechanisms. Guilt, embarrassment and shame often accompany anxiety." Disappointment might be similar enough to anxiety for your purposes, but I would say it's a lower-arousal form of negative affect (see the circumplex model of affect I mentioned in a previous answer). Disappointment may be a bit closer to guilt, embarrassment, or shame in terms of relatively neutral / non-specific arousal, but that's just an educated guess / intuitive judgment.
If disappointment $\approx$ these other negative emotions, let's return to Schimelpfening (2011): "An unconscious defense mechanism in which irrational or unacceptable behavior, motives, or feelings are logically justified or made consciously tolerable by plausible means." Clearly behavioral justification is definitional. "Post-decisional regret" – which sounds just about as similar to disappointment as any affect word I've mentioned yet – is avoided in the act of justifying a decision and prevented thereafter, according to cognitive dissonace theory (see Smith & Mackie, 2007; I may be interpreting loosely).
If this satisfies you that rationalization serves the purpose of preventing/avoiding/justifying negative emotion $\approx$ disappointment, the only remaining question (aside from your first one) is whether people use it. Burton (2012) offers some pretty good examples from literature and history, so as @rmayer06 said, the answer is "yes [more or less]".
Burton, N. (2012). Self-deception I: Rationalization: Human beings are not rational, but rationalizing animals. Psychology Today: Hide and Seek. Retrieved from: http://www.psychologytoday.com/blog/hide-and-seek/201203/self-deception-i-rationalization.
Schimelpfening, N. (2011). Rationalization. About.com: Health: Depression: Glossary. Retrieved from: http://depression.about.com/od/glossary/g/rationalization.htm
Smith, E. R., & Mackie, D. M. (2007). Social psychology. Psychology Press/Taylor & Francis (UK). Retrieved from: http://psypress.co.uk/smithandmackie/resources/topic.asp?topic=ch08-tp-01.