I don't regard my thoughtforms as a "defense mechanism", but more in terms of "creative self-development and self-expression." There is, I will admit, some degree of defense involved -- for instance, I sometimes let Anomia field personal attacks because she doesn't give a damn what anyone thinks -- but that is not the primary aspect.

Right now, my thoughtforms are helping to keep me sane & stable, since we're reaching a balance in which the opposites are finally beginning to cooperate with each other. In the past, I've experienced bipolar swings as I allowed one aspect or another to gain control. I don't think having thoughtforms causes negative personality traits to manifest, however, but rather that if the problems are already there, they may be expressed though that form. There are some psychological techniques which use interactions between inner "parts" to work problems out; for example, Inner Voice Dialogue, NLP, and Psychosynthesis.

If you would like to see an interesting example of someone else who "went plural", check out the blogger "Lucinda, et. al.", on h2g2: http://www.bbc.co.uk/dna/h2g2/U129960 (http://www.bbc.co.uk/dna/h2g2/U129960). Their essay "My Multiplicity" explains how the process got started for them: http://www.bbc.co.uk/dna/h2g2/A625123 (http://www.bbc.co.uk/dna/h2g2/A625123).