About the question: CH can rate entries, what if designers can do that also?
As a member of many many communities that also happen to have contest type threads or platforms. Every single one of the one that allow designers to vote as well end up getting abused & manipulated. Designers start voting for them self & asking their closest buddies at the community to vote for them. They also vote negatively for others to try & get a better shot at a win for them self & the prize money.
Not all designers do that & I would like to think a majority of people wouldn't resort to those tactics, however, they do happen in every place I see that has the feature. At the end of the day, it's just more accurate & productive to ONLY allow the person buying the design to rate it, if they want opinions, they ask associates, friends, family, & co-workers to view the contest thread. They then compile what they have been told by the people closest to them in life & adjust their rating / vote accordingly. I like it's better this way too. No need to change what isn't broken