[The bonus is that they will not commit more crimes for the duration of their sentence.]
Let's get this straight then.  What you advocated has no actual merit then?  Because we have to let prisoners out eventually and they can just commit another crime out of prison.  What difference is there in crime prevention except a matter of how soon they have another opportunity?

[a criminal's character is actually defined by the fact that he or she commits crime.]
Actually, no it is not.  This is kind of beside the point but a criminal violation may designate one as a felon but someone who violates the criminal code on a misdemeanor is still a regular citizen.  Someone who cannot justifiably be distinguished as a "criminal."  Especially if they seek and are granted sealing their case (since we don't have an expungement statute).  Then there are considerations for self-defense (with or without deadly force) as they are found to violate the law but are excused under the law.