Gregory
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Several reasons. First of all, unless your sentence is "life without the possibility of parole," you can, in fact, be paroled from a life sentence. I think that the more life sentences you have tacked on, the longer you have to wait to appeal to the parole board.
Second: suppose that you've been conviced of two counts of murder. Five years later, somebody else confesses to one of the murders. Now what? More specifically, what happens if you've been given one life sentence? It will be overturned, of course; you were arrested for murdering two people, and now it turns out that you didn't. You'll be held temporarilly while they arrange a new trial, for the murder you haven's been cleared of, but that happens if some of the important witnesses have died durring the last five years? If the prosocution can't find them? Even if they find all the witnesses, the crime occured five years ago; it's no longer fresh in their memory. What happens when they can't remember importand details? The whole thing will turn into a hideous ordeal; much better to just slap on two life sentences, and if you're proven innocent of one of the murders, no problem; they can just repeal one of the sentences and continue to hold you on the other.