Originally Posted by
rambam
haste may theoretically soft cap at that point for Flash of light, but it is still beneficial, it will slowly edge the spell towards being "instant" although never getting there
this allows you to respond better to spike damage, although not good when chain casting
also, haste can attempt to compensate for latency
additionally for holy light, if your lights grace drops off, there is occasionally an un-easy/un-comfortably long holy light, which during fights with a lot of spike damage can be deadly.
that said though, haste is like intel, it isn't the be all end all of stats for a holy paladin
while you will find haste on gear with MP5, it is good to understand that haste doesn't work well with mp5, because mp5 is static regeneration, where as crit scales with the number of spells you cast, and although crit doesn't really count as regeneration, more spell cost reduction, this should be considered when gearing.