Bucky Brooks posits that five-wide sets get defenders out of the box and spread the field enough that a shorter QB like Russell Wilson has it easier when reviewing the field (more passing lanes).
First, this at least partially explains the five-wide sets that we saw a lot on third-and-short last year. Combine the creation of passing lanes with the fact that we couldn't run the ball effectively on third-and-three-or-less last year anyway, and those 5WR sets make more sense.
Second, it was harder to run 5WRs without Percy Harvin. I liked having Lynch in on third-and-three-or-less because that leaves a legitimate PA fake. If Harvin stays healthy, you replace the threat of the run with Lynch with the threat of, say, a bubble screen with Harvin.
I feel much more comfortable about running 5WR sets this year because of Harvin.