I think the costs would outweigh the benefits. I think making absolutely sure there crime isn't repeated is overkill, since in many cases we can make it very unlikely that the crime is repeated, and not have to have the guy in prison for fifty more years. Also, I don't believe in "an eye for an eye", so I don't care if the sentence is more, less or equally severe as the crime. I just care about what's best for the society.
Surely you agree that some of the scumbags will repeat their crimes, no matter how rehabilitated they might be or pretend to be. Do you think that additional, preventable rapes and murders are an acceptable cost to save some money for the state and so that some convicted rapists and murderers who are actually reformed don't have to spend life in prison? To answer the reverse of that, yes I think locking up all murderers and rapists for life is an acceptable cost of making absolutely sure that none of them can hurt anybody again. Plus I think they deserve to rot in prison (and actually, I think that simply killing them would be better if we could be 100% sure that they were guilty).