Doesn't sockpuppetry occur when one poster uses another identity for deceitful purposes, usually to exaggerate his support or denigrate the quality of opposition?

Yes. That is precisely what we are talking about. You may argue that once the identities have been identified and can no longer be used simultaneously in the same conversation they are no longer technically sockpuppets. But that distinction doesn't seem terribly important.

There are prudential reasons for having multiple identities. It's like the protection of incorporation: it limits damage. If one identity goes down in flames after a particularly unpopular posting, it doesn't exhaust my "karma" capital.

Yes. There are incentives for using multiple accounts (whether for sock-puppetry, karma assassination or otherwise). I prefer the case where such practice is sufficiently discouraged that any perpetrators must at least go to the effort of acting differently---maybe by having different pet issues that they rant about rather than the same battle and same arguments with a different moniker.