If you're going to compete for a title in a division (regardless of how illegitimate a "title fight" it is), you should work within the established parameters which all other fighters in that division do. You shouldn't use your economic clout to force the creation of a situation which benefits only yourself.
If Pacquiao, or Mayweather, or anyone else, wants to fight for a title below 150lbs, then fight at welter. There are plenty of big, legacy-enhancing fights there that would mean a fuck of a lot more than beating a known cheat and possibly shot fighter who hasn't looked impressive in the ring in over 2 years.
Simply put, if you're too small for a division then either deal with it or don't fight at that weight. This catch-weight bullshit is absurd, regardless of which fighter is making these sorts of demands and it is only going to serve to mitigate Pacquiao's accomplishments at the higher weights.
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