this is just a test to see if sliva can handle fighting at LHW. but i give silva credit, because really this is a lose/lose situation for him. if he beats irvin, people are gonna say that irvin is a puff, and of course silva should have won. on the other hand if he loses, people are gonna bad mouth him for trying to move up, and how he got beat by a mid-level fighter at a higher weight class, and it could really diminish his reputation as one of the best pound for pound fighters in the world.
at the end of the day though its all about money i think. i'm sure that silva's competitive spirit has something to with it, but ufc knows that people are tired of watching silva destroy the MW division. if they can build upto fights with some of the top LHWs (liddell, shogun, wanderlei etc) those would be huge money fights, possibly some of the biggest ever in mma history. same reason there's been talks about penn vs. st. pierre and st. pierre vs silva. superfights make money!