Well at the time the Sharks needed to replace Nichol on the 4th line, and get help on the PK. Nichol did not do a good job replacing Malhotra the year he left. The PK was crap at the time and they had no one to take face-offs on the 4th line when Nichol was gone in 2012. Enter Burish, he fulfilled the roles perfectly.
Of course there was no way of knowing Desjardins would turn into a better player. Do you remember how CRAP he was his first 1-2 seasons.
Heck both McGinn and Desi in the lineup guaranteed the Sharks three things, they would take bad penalties, sometimes 5 minute majors, have great games followed by horrible games, and would always be caught out of position. Yet, they both figured out how to become better hockey players. But yea it really isn't Wilson's fault that Desi ended up taking Burish role. The overall price for Burish wasn't terrible (at the time), but the term was horrific.