Just want to start off by saying that i think some people put to much stock in simulations. don't get me wrong, they're amazing tools. but, they dont always reflect reality very well. as i stated months ago, having atleast 40% is a decent soft point for a few reasons:
Cooldown power, at 40% haste you receive the final tick of hammer of wrath (14th) during SW, sure the gain is small but you do not gain anymore casts, atleast optimally at 50%(which isn't even reachable with optimal gear). this fact combined with our greatly increased damage via mastery attacks(and crit!). mainly TV and four piece DS during SW. for every 600 mastery you pick up you gain an effective 2.5% damage increase during these periods. where additional haste gives us nothing.
outside of this however i can see haste being slightly superior, but again, not by much. the next reason is melee responsibility (hey, atleast this is reality right?) depending on your guilds strategy for many of these fights, and believe me, there are MANY ways to do almost every fight in siege, whats stats you pick will determine which will be best. if you constantly find yourself with uptime issues on the boss, haste will suffer more than mastery and crit contribution. how do you handle Nazgrim, Malkorok, Sha, Galakras, Spoils(oh yeah, i went there. adds) and heroic paragon burn phases? what about Heroic Siegecrafter belt duty? where mastery rets with 580 CDR trinkets make combat rogues look like trash? what about heroic Garrosh from the p3-p4 push where SW has a significant uptime during that short, crucial period? in a vaccume haste will likely end up on top with favourable conditions. but these don't exist.
unless your willing to take what a vast multitude of rets are now doing as wrong, without taking logs into account and understandiung why they're doing so, we've no discussion here.