Perhaps the main card will be all welterweight. UFC 146 had no heavyweights on the undercard, and 158 has Makdessi-Cruickshank announced already.
With so many events, having several high profile fights of one weight class would have advantages. Replacement fighters right there on the same card possibly, and also with more divisions the contenders can get lost in the shuffle and folks can have no idea who's in what weight class and what belt they are fighting towards. Putting several guys of the same weight on a card can help spell out and visualize the divisions and where people stand and so on
Obviously they can't do the 'all one weight class' format for every ppv but its nice when they can. It really builds excitement for future match ups. The person tuning into see GSP-Diaz will also get to see the next contender as well.
Seems like UFC learned their lesson from UFC 151 and the previously scheduled GSP vs. Nick Diaz card that turned into Nick Diaz vs. BJ Penn. They are lining up a ton of WW replacements in case Nick flakes out.