I agree, too many wild cards can hinder, rather than help. However, Ryan Harrison, is also 18, and at least in this case the wild card seems to have worked.
Harrison is ranked higher than Tomic, so I guess it could be argued Harrison is at this stage of his career a more capable player. But I think it may be more Harrison playing more challengers and earning points and practice that way. Harrison did not achieve his current position all though wild cards.
By the way, are there many other 18year olds ranked higher than Tomic?