Hit rating increases the likelihood that damaging spells will land on their target. Healing spells never miss, so hit is wasted points for a healer.
Haste is an elusive stat to comprehend in small quantities. It is easier to explain if you have a target in mind.
For a healer, let's say you would like to be able to cast four Flash Heals in the time it would normally take to cast three. With 1.5 second cast time unhasted, you would need to reduce that cast time to 1.125 seconds (a 33% time reduction). This would require a 1092 Haste rating (33.3 * 32.79). Hmmm, sounds like a lot of haste to achieve this one thing, doesn't it? Haste does shorten the GCD though, so it may be beneficial to have some haste to allow you to get more spells off in a shorter amount of time. Since it is % based, haste generally scales better with longer casting spells such as Gheal.
Haste can also be used to effectively manage your cooldowns. As Shadow, I believe you want enough haste to allow 2 Mind Flays in the time of a Mind Blast cooldown (I'm just starting to relearn Shadow atm.) With a cd on MB (talented) at 5.5 seconds and MF channelling for 3 seconds, you would want about 9% reduction in MF to 2.75 seconds to allow this. You would need 298 haste to achieve this.
These figures do assume L80, as haste does change as you level. Formulae and detailed explanation can be found
here As far as Spirit goes with Discipline, you shouldn't totally disregard it. Spirit is not as important to Discipline because it only serves the purpose of mana regen. Both Holy and Shadow receive some Spell Power from Spirit through talents and therefore benefit more from it. The regen cannot be completely ignored (especially with the upcoming nerf to Rapture), but an equal amount of mp5 can be used in your Disc kit. i.e. An item that has 20 Spirit is roughly equal to an item with 6 mp5 for Disc with respect to the IFSR, but Shadow would also receive 2 SP through Twisted Faith and Holy would get 5 SP via Spiritual Guidance.