I agree that for web design there are more entries in blind contests - CH should probably always choose blind and guaranteed options to get the best results.
This is not true of all contest types. The majority of contests are logo contests. Sometimes blind is better and sometimes open contest is better, depending on many factors. Really clients should be advised based on their brief and circumstances which options are likely to yield the best results for them.
As far as copying goes, designcontest works very hard to minimise copying as far as possible. Apart from banning cheaters quicker(which is already happening) I honestly could not think of any way to improve.