The prison system does not exist on the basis of an individual's likelyhood to re-offend - if it did, we could all claim one murder each and promise not to do it again.
Prison is a punishment appropriate to a variety of offences. It is designed to give the offender time to consider his actions, and to attempt to ensure that a repeat offence does not occur.
It also serves to show to society that crime is taken seriously, and is not simply ignored.
The attitude held by a large number of MP's that niggling little aspects of life like honesty, integrity, behaving like the majority of society in being a decent human being, and accepting the responsibility of representing others - are things they can simply ignore if they choose.
It is that level of breathtaking arrogance and self-importancec, and thievery, that has put Chater in court in the first place.
The notion that he should be let off because he is unliely to do it again (yes, I think you can safely say he will never be an MP again, so the expenses gravy train is an avenue of illegal and immoral theft that is sadly closed off to him) is frankly laughable.
MP's are instrumental in setting our laws and governing our society - they should be bound by its rules and punishments like the rest of us.