It's silly to equate a legal defense with a belief though.
I may not have the belief, personally, that spontaneous crimes of passion deserve a light sentence, but it would be malpractice for my lawyer to therefore not advance that argument if, god forbid, I was convicted of a murder. Whether I committed the murder or not, and whether or not I believe those sentencing guidelines should be extrapolated to wider society, as a lawyer you have an obligation to use every tool in your arsenal that can help your client. It has nothing to do with your own personal or political views on whether those tools should exist.
I may not have the belief, personally, that spontaneous crimes of passion deserve a light sentence, but it would be malpractice for my lawyer to therefore not advance that argument if, god forbid, I was convicted of a murder. Whether I committed the murder or not, and whether or not I believe those sentencing guidelines should be extrapolated to wider society, as a lawyer you have an obligation to use every tool in your arsenal that can help your client. It has nothing to do with your own personal or political views on whether those tools should exist.