Here's the real basis for why I think a move by Pacquiao to 135 will not be overly successful....
On paper, a move from 130 to 135 doesn't seem like that big of a deal, because after all, it's only 5 pounds. But when you look further into it, you'll see that making this move is actually much more than 5 pounds, because of the way these 2 divisions are currently constituted.
Pacquiao has been beating guys left and right at 130, but the opponents he's been facing haven't been true 130 pounders. In fact, they're all fighters similar to Pacquiao, in that they're former bantamweights and featherweights who have moved up.
Morales- fought at 122, 126, and 130, also went 0-2 at 135
Barrera- 122, 126, 130
Hector Velazquez- was natural at that weight, but has 11 losses on his record, it was just a "stay active" fight for Manny
Jorge Solis- fought mostly at 126. Two notable fights at 130 (Soto being the other notable opponent).
Oscar Larios- fought mostly at 122. In fact, in his fight previous to facing Manny, he fought Israel Vasquez at 122 and got TKO'd in 3 rounds. Moved up 8 pounds to face Pacquiao and went the distance.
I'm not criticizing Manny's level of competition, because I can't criticize anyone who's fought Morales and Barrera 5 times and went 4-1. I'm just pointing out that these guys were mostly blown-up 122 or 126 pounders. In all these fights, Manny was never at a natural size disadvantage. Therefore, it's not surprising that his relentess high-contact style would be effective.
135 is a different story.
Casamayor has never fought below 130. He's also a southpaw with a slick style, and his chin has survived 3 fights with Corrales (a very big puncher), and 1 with Castillo. So those factors add up to make him a difficult opponent.
The Diazs each started their careers in the 140-142 range, and now drain themselves down to 135 to compete there. If you look at them, they have a completely different body type than the guys who have been populating 126 or 130. As I said before, David Diaz is so limited skills-wise that I don't think any size advantage would be enough for him to beat Pacquiao.
But if Pacquiao were to face Juan Diaz, I think it would surprise people just how much bigger Diaz would appear in the ring. And the style matchup would probably lead to a very entertaining all-out brawl, at which point Manny would find himself for the first time trading punches with a bigger opponent.
I'm not trying to come off as anti-Pacquiao, because I love watching him fight. But history tells us that success over multiple weight classes is usually reserved for guys who fight nothing like Manny. Duran is the only real straight-ahead "puncher" who I can think of off the top of my head who won a bunch of different titles.