Saves using a prosthetic head I guess. I suppose with CGI they'd be able to make the body/head move and stuff up until the actual moment of decapitation in the same shot, rather than the old 'real Dooku with Anakin swinging sabres then cut to dummy Dooku having head lopped off' trick which has always looked obvious in similar shots in other movies.... Hopefully
A) The shot'll be moving so fast that the CGI wont notice...
OR B) they'll tidy it up a bit before release....
Or maybe C) The resolution difference makes it look a little crappier on our monitor screens. But then the rest of it looks cool, so maybe not...
🙁