1) The main reason haste was sought after for paladins in -4.2 was for adding ticks to Holy Radiance.
2) however since 4.0.6 mastery has been buffed enough.
3) that with Holy Radiance being changed to only last 3 second, making it incredibly impossible to add more than 4 ticks with Judgments of the Pure and not under the effect of raid buffs.
4) there is enough stat there on all of your gear to make Mastery actually give more throughput than haste, especially considering mastery affects the initial direct heal that Holy Radiance gives.
5) I definitely disagree that haste is the best throughput stat now. That being said, it's still pretty damn good... but mastery is better until you get to like, 30% or so.. why? because 30% bubbles from mastery takes ~2k mastery rating and having anything passed that to make a big difference.. e.g going from 30% mastery to 40% mastery is a big difference but 30% to 32% isn't going to be better than dumping those stats into haste to go from 1k haste to 1.2-1.3k haste.