Dark seekers and the television age

While watching television ads for I Am Legend, something occurred to me about the much-maligned CGI "dark seekers," the film's nocturnal mutant humanoids. While watching onscreen, they instantly seem cartoonish and break you out of the film's carefully crafted spell. But on television, for those few moments, they seem quick and exciting. That's vital, because the film is being sold as an exciting apocalyptic action-adventure, while the film itself is far less action-packed and even a bit of a - shhhhhhhh! - character study. So, the question I have is, to what degree was the choice made based on what would work theatrically, and to what degree was it based on what would work in advertisements?