You make a very good point - sometimes models and costumes, even imperfect ones - can look far more realistic than CGI.
Classic examples are:
The xenomorphs in Aliens vs the ones in Alien Resurection.
Storm troopers in the Starwars original trilogy - vs the clone troopers in the prequels.
Humans are so hard to get right using CGI. Even as advanced as the CGI was in Avatar - the smurfs just didnt look or move in a manner that was entirely convincing. As somebody else commented - the best CGI is the one you dont notice - many film makers simply havent got the balance right.
