Who will be the experts on this proposed site?
StackOverflow already works for beginners. Is there a need to isolate them to a separate site?
A site just for newbies is self-defeating, unless the members are extremely slow learners. Quite simply, nobody should be on such a site for very long before migrating to a "regular" site. It also has the fundamental problem that by its very nature the quality of the answers must be poor, as there would be no experts.
I don't believe a newbies site would be a significant drain on other sites because I don't believe it/they would ever have significant numbers of active members for the reasons already stated.