You can't completely ignore haste... 240 haste rating isn't enough for 2 x MF inside a MB CD for starters. Another reason is that some gear is going to come packaged with haste and trying to find an item with pure crit on it isn't going to be anywhere as good.
While there is going to be haste on a lot of gear it's not quite to the degree that we're going to get massive amounts of it. The BIS adds up to 420 haste rating with 839 crit rating (this includes gems, enchants etc and i'm choosing Eye of the Broodmother over Illustration of the Dragonsoul because I believe the pp listed for this item at shadowpriest.com is overally generous), so you can bias crit to some degree.
Another thing that's being forgotten is that haste will alow you to cast more spells in a given period of time... why would you want to ignore haste so you can cast only 6 MF ticks every MB CD when I can get gear that will allow me to cast 8 MF ticks every MB CD? The point is haste scales your dps just like crit does except haste doesnt rely on RNG.
The idea is you take the best items that will include both stats and you balance the 2... you should never forget one to stack the other.